THE 
Etiology and Prevention of Infectious Diseases * 
By GEORGE M. KOBER, A M., M. D., of Fort Bidwell, Cal. 
(Formerly of ■Washington, D. C.) 
[Reprint from the Virginia Medical Monthly, April, 1892.] 
It is in the field of infectious diseases where preventive 
medicine doubtless has and will continue to exert its great- 
est usefulness, and there is ample room when we contemplate 
the frightful mortality from these diseases. During the 
census year of 1880, consumption caused 91,270 deaths in 
the United States; scarlet fever, 16,388; diphtheria, 38,143; 
croup and allied diseases, 17,966; measles, 8,072; whooping 
cough, 11,064; enteric fever, 22,854; malaria, 20,231; and 
erysipelas, 4,275. 
Not many years ago, it was assumed that the infectious 
diseases were caused by a miasm, or a contagium. A miasm 
in the minds of those physicians constituted some hypotheti- 
cal matter formed outside of the system, which contami- 
nated the air and gained admission in respiration, and then 
produced pathological conditions. Moscati tried over a 
hundred years ago to demonstrate the character of malaria 
to the senses by condensing the moisture of the air over 
rice fields and marshy districts; and though certain organ- 
isms were found, all efforts proved useless to connect them 
with the disease as a causative factor, and thus the real 
nature of a miasm remained unexplained. 
Another class of physicians assumed that many of the 
infectious diseases were caused by a contagium, by which 
they understood a specific morbific material generated within 
* Read before the Medical and Surgical Society of District of Colum- 
bia, February 8th, 1892. 2 
the body of persons affected with a certain disease, and 
capable of producing the same disease in others. This con- 
tagium was presumed to be communicable by immediate 
or mediate contact, but they failed to isolate the various 
specific agents or to demonstrate them to our senses. 
About fifty years ago, Donne first demonstrated the 
presence of vibriones in syphilitic pus. In 1840, Henle 
ventured the opinion that the morbific matter of all infec- 
tious corpses was of a living organized character; and in 
order to determine whether the cadaver poison was produced 
by micro-organisms, he strongly urged to search for such 
organisms in the bodies of those affected with infectious 
diseases, and, if possible, to isolate them and establish their 
respective characters. This awakened a most earnest desire 
to determine whether or not organisms could be found in 
the bodies or excretions of infectious patients which might 
be the carriers or agents of thg respective virus. 
Very soon after this announcement, Davaine discovered 
the cause of anthrax; he found in the blood of animals 
suffering from splenic fever, invariably certain rod-shaped 
organisms, which, in 1855, were confirmed by Pollender and 
Brawell. These observers also demonstrated the fact that 
the blood of such animals inoculated into other animals 
produced the disease in question, and the blood of animals 
thus inoculated likewise contained the peculiar rod-shaped 
organisms. Pasteur, in 1877, finally proved that inocula- 
tion of blood, which had been freed from these organisms, 
failed to produce anthrax. This led to the natural conclu- 
sion that there was a causative relation between these rod- 
shaped organisms and the disease in question. 
In the beginning of the seventies, Waldeyer, Reckling- 
hausen, Weigert, and others, discovered a number of micro- 
parasites in the internal organs and wound secretions of 
various surgical infectious diseases; and in 1876 Koch pub- 
lished his researches on the etiology of splenic fever and 
anthrax, and presented the life history of the bacillus an- 
thracis, which is one of the many varieties of bacteria. 
The simple methods employed by this author in the 3 
detection and cultivation of disease germs resulted in the 
discovery of many other pathogenic bacteria, viz: the 
tubercle bacillus of Koch; the bacillus of glanders, by 
Schulz and Loeffler; the lepra bacillus, by Hansen and 
Neisser; the gonococci, as the cause of gonorrhoea and 
gonorrhoeal ophthalmia, by Neisser; the coma-bacillus of 
cholera, by Koch; the micrococcus erysipelatis as the cause 
of traumatic erysipelas, by Fehbeison; the tetanus bacillus, 
by Nicolaier; the staphylococcus pyogenes-aurens, albus, 
and citreus, the cause of suppuration, by Ogston and Passet; 
the lancet-shaped pneumonia bacillus as the cause of pneu- 
monia; the bacillus of typhoid, by Eberth, etc. The evi- 
dence as to the cause of puerperal fever is still conflicting, 
but points to the fact that it is due to the micrococcus 
erysipelatis; nor is it settled whether Loeffler's bacillus or 
Prudden's streptococcus is the cause of diphtheria. 
We may be disposed to smile with something less than 
contempt when we read of the importance attached to the 
microbicorigin of certain diseases; but Koch has laid down 
certain conditions, upon the proof of which alone it can be 
asserted that a particular microbe is the cause of a certain 
disease. They are: 
(1) The micro-organisms must be found in the blood, 
lymph, or diseased tissues of man or animals suffering from, 
or dead with the disease. 
(2) The micro-organisms must be isolated from the blood 
or tissues, and cultivated in suitable media outside the 
animal body, and these pure cultivations must be carried 
on through successive generations of the organism. 
(3) A pure cultivation thus obtained must, when intro- 
duced into the body of a healthy animal, produce the 
disease in question. 
(4) In the inoculated animal the same micro-organism 
must again be found. 
It has been shown, beyond a doubt, that some of the 
bacteria are the etiological factors of communicable diseases, 
and this is a strong argument in favor of those in which 
the proofs are not yet conclusive. For obvious reasons the 4 
inoculation of pure cultures cannot always be applied to 
man; but in diseases which are interchangeable between 
animals and men, the complete sequence of proof has been 
established, and the successful inoculation of certain patho- 
genic germs into healthy animals certainly warrants the 
assumption that similar results will take place in the human 
system. 
What are bacteria? Prudden tells us that they are so 
small that for a long time scientists wrere uncertain whether 
they belonged to the animal or vegetable kingdom; but it 
is now definitely settled that they are plants, and closely 
related to the algee. Under the microscope, we see that the 
bacteria or shizomycetes are made up of vegetable cells 
containing slightly granular matter and surrounded by 
a somewhat denser envelope; they are so pale and translu- 
cent that they can only be distinctly recognized by staining 
them. 
The bacteria may be spheroidal, ovoidal, rod-shaped, or 
spiral-shaped, and have been named according to their form. 
All spheroidal bacteria are called micrococci or simply 
" cocci." If they cling together and form chains they are 
called streptococci; if they are grouped together in a rather 
irregular manner, like a bunch of grapes, they are called 
staphylococci. 
Of the rod-shaped bacteria, the most common genus is 
called bacillus. If they are rather slender and delicate, they 
are called leptothrix. If they have the form midway between 
the round and rod-shaped bacteria, they are spoken of as 
cocci-bacilli or cocci-bacteria; the spiral or cork-screwed bacilli 
are called spirilla, and the curved bacilli are spoken of as 
coma-bacilli. 
Warmth, moisture, oxygen, and a certain amount of 
organic matter are essential for the growth of bacteria. 
When these conditions are present, the process of reproduc- 
tion is so rapid that a single germ, by growth and sub- 
division, may give rise to over sixteen millions of similar 
organisms within twenty-four hours; they multiply either 
by transverse fission or by spore-formation. 5 
Pathogenic bacteria may be present in all the media with 
which we come in contact, i. e., in the air, water, soil, food, 
the wastes of human life, in the excretions and secretions, 
in the blood, in the internal organs, and on the surface of 
the skin. Yet we must not assume that they are omni- 
present, for many of them select, after all, only certain 
localities or media. This is quite true of the plasmodium 
malaria, and likewise true of the bacillus of syphilis. The 
latter appears to select exclusively the human body as a 
suitable soil for its growth. 
So far, only the staphylococci of pus and the somewhat 
dubious bacillus of malaria have been demonstrated in the 
open air. 
In the air of rooms, the bacillus of pneumonia, of anthrax 
and tuberculosis, and the micrococci of pus and erysipelas 
have been isolated. 
In the water supply, the bacillus of cholera and of typhoid 
have been demonstrated. 
In the soil, the bacillus of anthrax, of typhoid, of tetanus, 
and malignant oedema have been found. 
In articles of food, the bacilli of anthrax and tuberculosis, 
and the staphylococci of suppuration have been found in 
the milk and meat of diseased animals. 
In clothing, the staphylococci of pus, the bacillus of an- 
thrax and malignant oedema have been demonstrated, and 
there is reason to assume that the germs of all diseases con- 
veyed in the air, can also be carried in clothing. 
It has also been shown that the walls, floors, and ceilings 
of our habitations may harbor disease germs. The pneu- 
monia bacillus has been found in the ceiling spaces, the 
bacillus of tetanus in mortar, and the tubercle bacillus on 
the walls, floors, bedsteads, and in fly spots. 
The surface of our body harbors many bacteria, both 
harmless and pathogenic. They are especially numerous 
in the armpit, between the toes, and the dirt of finger-nails. 
Multman demonstrated no less than seventy-two varieties 
of bacteria in nail dirt. Of the pathogenic forms, the 
micrococci of pus and the tubercle bacillus have been 
demonstrated in nail dirt. 6 
The pyogenic cocci have also been found in the pustules of 
impetigo and sycosis, and the bacillus of syphilis in the 
secretions of syphilitic ulcers and condyloma. It is quite 
likely that the bacilli of cholera and typhoid will be found 
on the skin of persons soiled with dejecta, and that the 
scales of the epidermis contain the carriers of the acute 
exanthemata and of erysipelas. 
In the mouth and saliva, there have been found four dif- 
ferent bacteria, capable of producing septicaemia in animals; 
also the streptococci of diphtheria and the bacillus of tuber- 
culosis and pneumonia, not to mention the oidium albicans, 
which belongs to the fungi, as the cause of thrush. 
In the mucus of the respiratory passages, the bacilli of 
pneumonia and tuberculosis have been isolated; also 
Frankel's and Weichselbaum's pneumoniacocci. In vaginal 
mucus, the oidium albicans, the staphylococci of pus, and 
the streptococci of diphtheria have been found. 
In the intestinal contents, the bacillus of cholera and 
typhoid, and it is assumed that the carriers of dysentery 
and tuberculosis are also present. 
The blood and internal organs of healthy individuals are 
free from micro-organisms. In disease, the bacillus of 
typhoid has been found in the blood and spleen; the bacilli 
of anthrax and tuberculosis have been demonstrated in the 
blood and various internal organs; the same is true of the 
streptococci of diphtheria. 
Normal urine has been found to be quite free from germs, 
but bacteria have been demonstrated in certain diseases of 
the bladder and embolism of the kidneys. 
The cutaneous perspiration is believed to be free from 
microbes; if found, they are probably derived from the 
skin. The exhalations of healthy and diseased persons, even 
the breath of consumptives, have been found to be free 
from bacteria. But more remains to be done in this direc- 
tion, as there are many infectious diseases which are doubt- 
less in their first stage communicated by the exhalations of 
the patients. 
In this connection, it is well to remember that, in addi- 7 
tion to disease germs, there are many so-called harmless 
bacteria. The latter [are so abundant and common that, 
apart from existing in the air, water, soil, and food, they 
are also found in the mouth and digestive tract of man, in 
the nose, in the respiratory passages, in the internal meatus. 
The faeces of the healthiest subjects contain numerous fer- 
ments and bacteria, and even the meconium of the new-born 
has been found to contain bacteria. 
One of these bacteria, the bacillus coli communis, was con- 
sidered heretofore a harmless inhabitant of the colon, but 
the recent studies of Rodet and Roux render it probable 
that the bacillus of Eberth is nothing more than a modifi- 
cation of the bacillus coli communis, and that the latter 
acquires its virulent features outside of the body. And 
this opens a question of much practical interest, Whether 
bacteria may be benign in one locality and malignant in 
another, or whether the property of infection is an adventi- 
tious one, depending upon the soil in which the organism 
grows, or inherent in its substance? Of this, more will be 
said hereafter. 
Having indicated, briefly, where the disease germs have 
been found, it will be readily inferred that the air, water, 
soil, and articles of food are the principal media for their 
dissemination. Apart from these, clothing, bedding, and 
personal effects are common means of spreading infectious 
diseases. Man himself is often the cause of spreading 
disease germs. This is true of many mild cases of typhoid 
fever, and other diseases of a mild type, not to mention the 
spread of scarlet fever, measles, small-pox, etc., by conval- 
escents and infectious corpses. 
Perfectly well people may carry the germs in their cloth- 
ing, or about their person, and infect some distant locality. 
One of the severest scarlet fever epidemics I ever encoun- 
tered was due to a man who came after me on horseback, 
some sixty-five miles, to see some scarlet fever cases. He slept 
that night with one of his little nephews, and in less than 
ten days that boy developed scarlet fever, which subse- 
quently invaded almost every family. 8 
Insects, especially flies, are doubtless frequently the cause 
of spreading disease germs; they are usually present where 
the existence of germs may be suspected, and feed on the 
material likely to contain them. As a matter of fact, Cellir 
of Rome, in 1888, demonstrated that the bacilli of tubercu- 
losis, of anthrax, and typhoid, as well as the micrococci of 
pus, retain their virulence and power of propagation after 
passing through the intestinal tract of flies. Spill mann 
and others have demonstrated that flies, which were caught 
whilst feeding upon the sputum of consumptives, contained 
invariably vital specific bacilli, and Hoffmann not only 
confirmed these observations, but also found them in the fly 
spots on the walls of rooms of phthisical patients. 
As already indicated, bacteria require for their growth a 
suitable soil; they need moisture, carbon, nitrogen, and 
mineral matter, and quite a number also oxygen. The ma- 
jority cannot exist in highly acid nutrient media, but'pros- 
per best in neutral and faintly alkaline media. It has also 
been demonstrated that they vary considerably with regard 
to their pabulum; some flourish in certain media, where 
another species finds it hard to live. 
The majority of pathogenic germs develop best at a tem- 
perature between 28° and 68° F. The vitality of some is 
destroyed at the freezing point, whilst others remain active 
even in ice for long periods. A temperature of 250° is 
quite likely to destroy all varieties of bacteria and their 
spores. 
In the cultivation of bacteria, some interesting observations 
have been made. It would appear that wherever varying 
forms of bacteria are growing together a struggle for exist- 
ence begins. The hardy and rapidly growing bacteria 
struggle with their weaker neighbors, and the weaker, as 
usual, go to the wall. 
Thus it has been observed, that the bacteria of fermenta- 
tion and putrefaction furnish a material which poisons 
neighboring pathogenic bacteria. But such an antagonism 
does not always exist, for many varieties appear to grow to- 
gether, whilst others prepare the way for a succeeding race, 9 
and often one species adapts itself to the conditions which 
brought about the extinction of its predecessors.-(Prud- 
den.) 
Bacteriological studies have also shown us that, as the 
bacteria grow, they set free various chemical compounds- 
the result of their own life processes-chief of which are the 
ptomaines; they also liberate some of the amido-compounds 
of the aromatic series, and ammonia, taurin, fatty acids, 
hydrogen sulphate, carbon dioxide, oarburetted hydrogen, 
and other stinking gases, not yet determined. 
The subject of ptomaines is to us of special importance, 
because it is believed that these alkaloids, closely resembling 
the vegetable alkaloids in chemical and pathological prop- 
erties, produce the real mischief in the human system. 
Quite a number of ptomaines have been discovered, and 
we know to-day that the cadaver poisons, such as cholin, 
neuridin, neurin, cadaverin, putrescin, etc., are the products of 
bacteria. Ptomaines have also been demonstrated in de- 
composing milk, cheese, meat, and fish. 
But most important of all, they have also been discovered 
in the culture fluids of certain pathogenic microbes. Thus 
cadaverin, first demonstrated in the human cadaver, has been 
found in connection with putrescin in pure cultures of the 
cholera bacilli. And it is believed that the symptoms of 
cholera are produced by these poisons; for when introduced 
into animals, violent inflammation and necrosis of the in- 
testinal mucous membranes results. 
Typhotoxin has been obtained from pure cultures of the 
typhoid bacillus; this alkaloid, when injected into animals, 
produces inability to control the voluntary muscles, and 
excites increased flow of the salivary and intestinal secre- 
tions. 
Another ptomaine, called tetanin, has been demonstrated 
from the cultures of the tetanus bacilli. This alkaloid, when 
injected into animals, produces clonic and tonic spasms-in 
fact, the symptoms of tetanus. The recent researches by 
Koch as to the discovery and effects of tuberculin are too 
well known to require special notice. 10 
The admirable methods of Brieger, employed in the de- 
tection of ptomaines, will doubtless succeed in demonstrat- 
ing that such products are formed in all pure cultures of 
pathogenic bacteria, and this would, of course, materially 
enhance our knowledge as to the deleterious effects of these 
compounds on the system. 
Indeed, it would seem that the time is not far distant, 
when we can say, that certain bacteria invade the system. 
These develop, under favorable conditions, in sufficient 
number to produce, by their own life-processes, certain pto- 
maines, which, in turn, cause a train of toxic symptoms not 
unlike those of the vegetable alkaloids. When we consider 
the peculiar idiosyncrasies of individuals to some of the 
vegetable alkaloids-morphine and quinine, for instance- 
we need not wonder at the variable effects of ptomaine in- 
toxication in infectious diseases. 
There is much reason for believing that the urticaria and 
gastro-intestinal symptoms produced in some persons after 
eating shell fish, etc., are due to the production of ptomaines 
in susceptible subjects. It is also held that ptomaines may 
be formed in perfectly healthy individuals in the intestines, 
under the action of the bacteria of putrefaction, and the in- 
dol, phenol skatol, and kreosol, found in the urine, are 
really products of these processes. 
There is evidence to show, that one ptomaine may act as 
a counter-poison to others; we know, at least, that some of 
these products destroy the very organisms which generated 
them. So, for example, lactic acid, which is the product of 
the bacillus acid lactici, destroys that same bacillus in a one 
per cent, solution; and this, perhaps,explains the efficacy of 
lactic acid in the treatment of cholera infantum. Grawitz 
has shown that cadaverin in certain concentrations arrests 
the growth of the pyogenic micrococci. We also know that 
some species of bacteria die in the same culture fluid, while 
others live, which possibly furnished the material to poison 
their neighbors, and the presence of phenol, skatol, and 
kreosol, which are products of putrefactive micro-organisms, 
retard the growth of these same germs. 11 
Many persons still believe in the spontaneous generation 
of bacteria, and naturally hold that, if disease germs can- 
not originate de novo, all efforts in prevention must prove 
useless, because the divine law of death has decreed other- 
wise. Whilst the experiments of Pasteur, Tyndall, and 
others, have shown that every living thing-be it man, 
beast, plant, or cell-must come from some pre-existing liv- 
ing thing, clinical experience points to the occurrence of 
many infectious diseases, which cannot always be traced to 
a pre-existing cause; and the question naturally arises. 
Where did the germs come from ? 
In 1886, I was called to treat three young " cow-boys," 
who had been alike exposed to the process of catching cold. 
One developed facial erysipelas; the second, an abscess of 
the thigh, following a slight saddle-bruise; the third, a case 
of typho-malarial fever. Here we have three young men, 
of average good physique, exposed to a chilling of the gen- 
eral surface, and consequent suppression of the cutaneous 
function, causing a retention of the effete matter in the 
blood, which, in turn, supplied a suitable pabulum for the 
development of septic germs. Still, it seems to me quite a 
mystery, why, in one case, the micrococci of erysipelas; in 
another, the staphylococci of pus; and in still another, the 
etiological factors of typhoid fever and malaria, should have 
been developed. 
In tracing the family history, I found that the mother of 
my erysipelas patient had died from that disease in one of 
her confinements. Such cases are especially calculated, in 
the absence of a pre-existing case, to suggest the possibility 
of an auto-genetic origin of disease germs. 
In our present state of knowledge, we can only explain 
such cases by assuming that disease germs may gain admis- 
sion into the system at all times, but can only multiply in 
sufficient numbers to produce mischief when the blood, from 
any of the numerous causes, is charged with an excess of 
waste matter. If disease germs are not omnipresent (and I 
do not believe they are), we must assume that auto-infection 
can take place. It is quite possible that the ordinary so- 12 
called harmless bacteria acquire virulence by suitable food 
and environments; and the retention of leucomaines and 
extractives in the blood, whose elimination has been inter- 
fered with, may furnish the requisite food for such trans- 
formations within the body; and that soil pollution and un- 
sanitary surroundings may do so outside of the body. A 
disease thus developed could, of course, be spread in the 
usual manner. 
In my Report to the State Board of Health of California in 
1886, I attributed certain cases of typho-malarial fever and 
erysipelas to autochthonous septicaemia, and presented a 
strong argument in favor of the de novo origin of diphtheria. 
At all events, I adduced evidence to show that the same 
polluted soil and unfavorable sanitary surroundings were 
capable of developing different disease germs. 
We know that there are mild and malignant epidemics 
of scarlet fever, measles, etc., indicating at least a difference 
of degree in virulence. We also know that the virus of 
certain infectious diseases may be attenuated; and there is 
no special reason why certain harmless bacteria should not 
assume pathogenic action by changed environments within 
or without the body. 
But whatever the origin of disease germs may be, I be- 
lieve they would never produce deleterious effects if the 
human system were at all times fitted to resist them. 
We certainly know that invasion of the microbes alone is 
not sufficient to produce the disease, because this can take 
place often enough. If, therefore, disease results, there must 
have been a state of the body which rendered it. peculiarly 
liable to be affected injuriously by the respective morbific 
agent, and this we call predisposition. 
Now, what constitutes this predisposition ? Is it a weakness 
of the organism, a diminished power of resistance, or a pe- 
culiarity of the tissues-more especially of the tissue cells 
-which places them at a disadvantage in their struggle 
against the invasion and effects of these germs? Is it the 
addition or subtraction of a certain something in the blood, 
which furnishes the proper food for their growth? These 13 
are difficult questions to answer, but we will sum up the ex- 
isting views on this subject. 
In the first place, it is suggested that this predisposition 
in many persons amounts to only simple lesions of the skin 
and mucous membrane, which would favor the invasion of 
the germs. Thus it is held that the peculiar susceptibility 
of persons, suffering from whooping-cough and measles, to 
tuberculosis, is due to the catarrhal condition and altera- 
tion of the epithelial layer of the respiratory tract, which 
favors the invasion of the tubercle bacilli; for similar rea- 
sons the delicate mucous membranes of anaemic and scrofu- 
lous subjects are believed to favor the invasion of these 
germs. This is highly probable, when we consider that 
only the delicate mouths of children are susceptible to the 
effects of the oidium albicans. Loeffler has also shown that 
the vagina of young guinea pigs favors the inoculation of 
his diphtheria bacilli more than that of older animal^. 
Functional derangements of the stomach appear to play an 
important r6le in predisposition. Thus we know that nor- 
mal gastric juice destroys the bacilli of cholera; but if the 
fluid is only faintly acid, neutral, or alkaline, this protection 
ceases, and the germs will reach the intestines and prolifer- 
ate. This is doubtless true of many other germs. 
For a time at least it was believed that the leucocytes 
assumed the role of defenders of the body against bacterial 
invaders-that, according to Metschnikoff, they are phago- 
cytes in this, that they either swallow and digest the bacte- 
ria, or surround them so closely as to cut off their oxygen 
and food supply, and thus kill them. According to this 
theory, which was based on the fact that many dead bacilli 
were found inside the bodies of leucocytes, an animal whose 
white blood-cells can successfully battle with and eat up a 
given species of bacteria, enjoys immunity from its delete- 
rious effects; if, on the other hand, they are not strong 
enough to resist them, the battle is lost and the way is open 
for the spread of the infecting germs. 
Now, whilst it cannot be denied that bacteria have actually 
been found in the bodies of leucocytes, it does not follow 14 
that they were taken up alive and destroyed by these cells; 
and until this is shown, no proof exists that their action to- 
wards the bacteria differs in any way from that towards 
other dead matter which they absorb or transport. This 
has been well presented by that indefatigable bacteriologist, 
Dr. Prudden, of New York. 
Indeed, recent experiments appear to indicate that after 
all, "it is the blood serum which possesses in different de- 
grees in different animals and in varying potency with the 
different bacterial species, a most marked germicidal power; " 
and this same observer has proved that a similar germici- 
dal power resides in fresh, human, non-inflammatory trans- 
udations. In his opinion, " this power is not directly asso- 
ciated with the formed elements of the blood or transudates, 
but is in some way dependent upon their albuminoid con- 
stituents, and that this singular and apparently most signifi- 
cant capacity of the body fluids is intimately associated 
with that complex condition which we call life." 
Whatever the exact rdle of the blood may be, the influ- 
ence of a healthy condition of this fluid cannot be ques- 
tioned. If healthy blood offers a natural defence, impure 
blood would constitute a prominent predisposing factor; 
and the average layman may after all not be very wrong 
when he talks to us about his "blood being out of order." 
We know from clinical experience, that bad food, impure 
air and water, and the absorption of putrid gases formed in 
the intestinal tract, general mal-nutrition, physical and 
mental exhaustion, fever, anxiety, etc., predispose to a num- 
ber of infectious diseases; and it is not unreasonable'to as- 
sume that these factors, apart from producing an alteration 
of the blood, also tend to lower the vital powers of the sys- 
tem, and necessarily diminish the power of resistance to 
bacterial invasion and their products. Wyssokowitsch's 
experiments have shown that the same bacilli, which un- 
der normal conditions of the blood were rapidly destroyed 
under the influence of a ptomaine poison, at once began to 
manifest their pathogenic effects. 
In the light of clinical experience, the existence of an 15 
individual predisposition to infectious diseases cannot be 
questioned. We also know, that this may be congenital and 
acquired; and in our present state of knowledge, we may 
assume that a vulnerability of the tissues and tissue cells 
exists, which renders the system peculiarly susceptible to 
the invasion of certain germs, and the particular form of 
poison evolved during their life processes. 
It is no more difficult to account for inherited vulnerable 
anatomical elements to certain germs and their toxic pro- 
ducts, than to explain the hereditary transmission of phy- 
sical and mental peculiarities, and the many idiosyncrasies 
to drugs, etc. 
•Now, whilst we find a natural predisposition to certain 
diseases, we also observe a natural immunity from certain dis- 
eases. This has been ascribed by some to the fact that the 
avenues for the invasion of the microbes are so well pro- 
tected that they cannot enter the system, or that the blood 
and character of the tissues render it a poor soil for the 
proliferation of the disease germs; but these are unsatisfac- 
tory explanations in many instances. We know there are 
mild and malignant cases of scarlet fever, walking cases of 
typhoid, and cases of a profound type. We also know that 
some systems'are peculiarly susceptible to the physiolog- 
ical effects of certain drugs, and others are not. For all 
such reasons, I am tempted to accept Arlong's explanation, 
that immunity is due to the inaptitude of certain organisms 
to feel the effects of ptomaines evolved by the microbes. Ac- 
cording to his explanation, germs of infectious diseases may 
exist within the body, and the disorder they give rise to may 
be absent; for infectious disease does not exist merely because 
some noxious micro-organisms have taken their abode in 
the system. They exist because functional or structural 
troubles are brought about through the agency of pto- 
maines. Of course, this explanation does not apply to all 
infectious diseases, for in some instances, one attack affords 
no exemption from a subsequent attack; indeed, the sus- 
ceptibility is rather increased than diminished. This is true 
of malaria, gonorrhoea, croupous pneumonia, and erysipelas- 16 
The question of practical interest as to how the disease is 
brought about by the bacteria, is next in order. Some of the 
germs evidently remain, for a time at least, at the point of 
invasion and vicinity; here they grow, and as they grow, 
evolve the so-called ptomaines, which being irritants, often 
cause an inflammation, and this as in erysipelas, malignant 
oedema, cholera, and some forms of pneumonia, may go on 
without suppuration. Other bacterial forms cause the white 
blood cells to gather about the parts, and as they accumu- 
late more and more, the leucocytes die, and we have in- 
flammation with suppuration, as in abscess, furuncles, im- 
petigo, and sycosis; other forms of bacteria cause inflam- 
mation with necrosis, as in diphtheria, hospital gangrene, 
etc. 
Another class of germs proliferate at the point of inva- 
sion, and subsequently invade the deeper tissues. This is 
probably true in tuberculosis. The bacilli of tetanus appear 
to proliferate at the point of invasion, and spread along the 
nerve trunks, and finally enter the blood. The germs of 
syphilis and of leprosy-are at first localized, and finally in- 
vade other parts. The virus of hydrophobia appears to de- 
velop first at the point of invasion, extends along the nerve 
trunks, and subsequently invades the central nervous sys- 
tem and blood. 
In some diseases, the pathogenic germs are transported 
through the lymphatic spaces and vessels; in others, the 
invaders enter the blood, multiply there, and either remain 
or are deposited in different organs, or in all parts of the 
body; this may be said of the germs of anthrax, of ty- 
phoid, of acute miliary tuberculosis, and the various acute 
exanthemata. 
An exact classification would be entirely premature. The 
most we can say at present is, that the effects of pathogenic 
germs may be local and constitutional, the result of the ab- 
sorption of their toxic products. Take, for example, a case 
of malignant scarlatina, where a child in the midst of per- 
fect health is suddenly seized with vomiting, perhaps vio- 
lent diarrhoea, with a remarkably frequent and small pulse, 17 
and such sudden collapse that the pulse disappears within 
eight to twelve hours, the face and the extremities become 
cold, and death occurs in coma within twenty-four or thirty- 
six hours, without any eruption having appeared. Such 
symptoms point to paralysis of the cardiac centre; and if 
these and all the various shades of brain symptoms in in- 
fectious diseases are not the result of ptomaine intoxication, 
it will be difficult to account for them in any other manner. 
If it be true that infectious diseases are produced by the 
invasion of bacteria, their subsequent development and the 
poisonous compounds evolved during their growth, the next 
question to be disposed of is, How and under what circum- 
stances do these processes terminate f 
We have seen that under certain circumstances the accu- 
mulation of disease germs and their products may be so great 
in the capillaries as to destroy the vitality of the part by 
cutting off the blood supply. Now this may be a purely lo- 
cal condition; but let us figure to ourselves all the blood of 
the body charged with an organic poison, the capillaries ob- 
structed with the invaders; and should this perchance take 
place in the same vital organ, we can at once appreciate how 
either or both of these conditions may produce death. If 
death does not take place under such circumstances, it is 
because at this critical moment something occurs which 
renders further proliferation of the germs and ptomaine 
production impossible, and recovery ensues, provided path- 
ological lesions do not prolong the disease. 
It. was at one time assumed that the bacterial invaders 
were eliminated by the kidneys and other emunctories of 
the body; but Wyssokowitsch's experiments indicate, that 
in animals, at least, they are not thrown off by the kidneys, 
unless these organs are themselves the seat of disease. Nor 
can we ascribe the destruction of the disease germs to the leu- 
cocytes ; for it seems to me, if they were not vigorous enough 
to cope with them at the onset, it is not likely that their 
phagocytic power increased during the illness. 
There remain but a few explanations. One is that recov- 
ery ensues because of the exhaustion of certain substances 18 
essential to their growth ; in brief, they die for want of suit- 
able food. The second explanation, which is extremely 
plausible, is, that the very poison evolved by the bacteria 
has accumulated to such an extent as to prevent their own 
growth, and they die self poisoned. There is nothing 
strained in this theory, as culture experiments indicate this 
possibility. Since some bacteria do not flourish with a 
high temperature, it has been suggested'that a continuous 
fever heat may aid in their destruction; but this seems to 
me the least acceptable explanation. 
I have consumed much of your valuable time in the pre- 
sentation of our knowledge on the germ theory of infec- 
tious diseases. Many points are still obscure and require 
explanation. 
Whether internal medication will ever succeed in render- 
ing pathogenic germs innocuous remains to be seen. Whilst 
we possess remedies of decided value for local use, it is 
doubtful whether we can employ these agents in sufficient 
concentrations to be of service in internal medication. 
A 
Nevertheless, it is possible, that we may be able to diminish 
their virulence; and we can at least place the system in a 
better condition to resist them, and perhaps, after awhile, 
some appropriate antidotes will be found for the various 
bacterial forms and their toxic products. 
In the meantime, the bacteriologist deals with living or- 
ganisms. In place of hypothetical agents, like miasms and 
contagia, he has isolated these germs, demonstrated their 
morphology and biology, and to what extent they are in- 
strumental in the production of disease, and how and by 
what means these organisms may be destroyed outside of 
the body. All of these studies indicate that much may be 
done by efficient and thorough disinfection in the preven- 
tion of infectious diseases; and it is in this field bacteriol- 
ogy will achieve its greatest triumphs, and humanity derive 
its greatest blessings.