Updated: Jul 24
While in modern times yellow fever is confined mainly to Africa, and to a lesser extent, to South and Central America, there was a time when it was feared all over the world. The first major yellow fever outbreak hit Philadelphia, the largest city in the U.S. at the time and the nation's capital, in the summer of 1793 and peaked in October of that same year. Out of 45,000 residents, 5,000 died from yellow fever and another 15,000 fled the area. Yellow fever is notable for the jaundice skin color it causes (which malaria also causes by the way) but it also causes fever, vomiting, internal bleeding and death in some cases.
The disease was transported by ship and it was carried both by the ship's passengers who were infected, and by mosquitoes from Africa. The mosquitoes were probably more dangerous than the infected people because mosquitoes native to North America are unlikely to be able to transmit the virus. The Aedes genus of mosquitoes from Africa would have been the most likely carrier at that time though there are a few other varieties of mosquitoes that can carry it. Yellow fever can't be easily extinguished in the world (or extinguished at all) because of its ability to jump from humans to nonhuman primates and vice-versa (see image below). Certainly it wouldn't be possible to vaccinate all of the arboreal primates of Africa.
So why do we not have yellow fever in the U.S. any more?
The answer is that we can have both the mosquitoes and the outbreaks in the U.S. but we just fortunately haven't had too much of an issue with tropical diseases generally. Yellow fever is limited by climate, with warm, wet climates being more suitable to its transmission. It is also limited by mitigation measures that reduce local mosquito populations and by vaccination efforts.