To examine his next disease, malaria, Quammen turns from the relatively recent present to the past—specifically, to a British doctor named Ronald Ross who would later pioneer the study of malaria and develop an interest in infectious disease more generally. Ross grew up in India and returned to his native England to become a doctor. Though he had artistic and mathematical ambitions, he went on to successfully practice medicine. He was posted to Madras, India, where he first began to ponder the causes of malaria and took several blood samples from malaria patients.

Ross’s progress with understanding malaria was somewhat hampered by his remote location. Eventually he learned about a French doctor who “had discovered tiny parasitic creatures (now known as protists) in the blood of malaria patients. Those parasites […] caused the disease” (128). Ross’s later experiments convinced him this conclusion was correct. Ross’s own research is notable, Quammen argues, because he studied malaria in birds to understand its operation and “life cycle.” His other major intervention was to apply his mathematics background to analyzing malaria.

Though viruses might seem far more biological than mathematical, mathematical principles illustrate how diseases successfully persist in populations. One particularly important concept is known as “critical community size” (129): When the population of possible hosts is too low, the disease can’t persist and will “die out.