It will likely come as a surprise that the third viral vaccine created was against yellow fever, a disease that is virtually unknown to the American public in the twenty-first century. While yellow fever remains endemic in parts of South America (particularly Brazil) and Africa, North America hasn’t seen a case in over 100 years. Given its long absence, it’s hard for contemporary society to imagine that yellow fever was once the most feared illness in the Americas, throwing entire cities into a panic when cases materialized. Unlike rabies that came from animal bites, yellow fever was mysterious as it was never certain when or where it would appear. In the United States, it was known as a disease that typically began in the spring and early summer and persisted until the first frost, but its occurrence was haphazard. Cities might have a terrible outbreak one summer and then not see cases again for many years. Theories abounded about where it came from and how it spread, most of them completely wrong, and it wouldn’t be until the early 1900s that this disease was finally understood. The inability to fathom the source of this disease, predict when and where it might strike, or take effective preventative measures contributed to the fearfulness toward yellow fever. It was as if this illness was a random curse that descended without warning upon ill-fated communities. If cases did begin to appear, many city residents would immediately flee to escape the disease and avoid being trapped in areas that were often quarantined to prevent further spread. Given that there were no effective treatments and the fatality rate could be as high as 50% (Patterson 1992), fleeing seems like a very prudent response.