From late May to early August 1934, Dr. Sawyer again visited yellow fever control operations in Brazil. This included a ten-day trip to the Mato Grosso region to check on several puzzling outbreaks of yellow fever in rural areas, and led to the recognition (by Dr. Fred Soper and others) of a new infection pattern: jungle yellow fever. As Sawyer also noted in this letter, this visit was also characterized by the extended use of aircraft in making the rounds.. NOTE: The original letter is written on onionskin paper.
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