Kids with yellow fever in Mississippi about 1870.
See how all the kids are white? They weren't letting black kids
into the hospital.
The virus that causes yellow fever probably evolved from an earlier virus that didn't make people sick. The first cases of yellow fever may have been in East or Central Africa, but people in West Africa were catching yellow fever by 1600 AD. When people from West Africa were forced to travel to North and South America in the 1600s to work as slaves, apparently some of them had yellow fever, so the virus came with them.
You caught yellow fever when a mosquito bit an infected person and then bit you. If you caught yellow fever, you got a sudden fever and headache. Most people slowly got better, but about a third of the people who caught yellow fever got bad cases. If you got a bad case of yellow fever, you became extremely tired and began bleeding into the skin. You got a slow heartbeat, back pains, and vomiting. Then your liver stopped working, and your skin turned yellow with poisons that your liver couldn't filter out (That's why it's called yellow fever). About half the people who got bad cases, or one out of every seven people who caught yellow fever, died.
When mosquitoes bit these African people, they spread yellow fever to other people. In 1648, yellow fever killed a lot of people in the Maya country. In 1685, it killed a lot of people in Brazil. In 1793, an epidemic of yellow fever killed a lot of European colonists in Philadelphia. By 1822, after a serious epidemic in New York City, northern port cities began to quarantine ships as they came into port to stop yellow fever, and there were no more epidemics in the north. But many people continued to die of yellow fever in the American South and in the Caribbean islands, where there were a lot of mosquitoes to spread the disease.
In 1881, Carlos Finlay, a Cuban doctor, suggested that mosquitoes might be spreading yellow fever, and when experiments showed he was right, people began to work hard to get rid of the mosquitoes. This effort stopped most of the yellow fever epidemics. In 1937, Max Theiler, who was originally from South Africa but was working in New York, invented a vaccination against yellow fever. Because of the vaccination, and programs to control mosquitoes, not very many people catch yellow fever anymore, though there's still no cure for yellow fever if you do get it.