The Forgotten Plague That Almost Destroyed America

In 1793 when mercury and bloodletting were used to treat diseases, and we didn't even have germ theory, it would have been a lot to expect people to understand acquired immunity. Or that diseases could be spread from contaminated water or by mosquitos instead of by other sick people. When yellow fever struck Philadelphia that year, at least 5,000 people died, which was 10% of the city's population. PBS tells us how that played out. 

We still don't have a cure for yellow fever, but we have better supportive treatment, and a vaccine that is recommended before you travel to a region where the disease is endemic.