Yellow Fever

Many of the common illnesses in the United States, including the common cold, diarrhea, and influenza are spread via direct passage of the causative virus between human beings. Yellow fever, however, cannot be passed from one human being to another. Rather, the virus responsible for yellow fever is transmitted through an intermediate vector—a mosquito—which carries the virus from one host to another. The hosts of yellow fever include both humans and monkeys. The cycle begins when an infected monkey is bitten by a tree-hole breeding mosquito. This mosquito acquires the virus, and can pass the virus to any number of other monkeys which it may bite. When a human is bitten by such a mosquito, the human may acquire the virus. In the case of South American yellow fever, the infected human may return to the city, where an urban mosquito (Aedes aegypti) serves as a viral vector, spreading the infection between humans.

The host-vector-host cycle of yellow fever was first described by Walter Reed (1851-1902), the military surgeon for whom the Walter Reed Medical Center in Washington, D.C., is named. Reed was commissioned by the United States government to study yellow fever transmission. Reed's discovery of the mosquito as an intermediate vector led to improved control over the spread of the disease, ultimately allowing the building of the Panama Canal in an area prone to yellow fever epidemics.