Unlike malaria, yellow fever has failed to stimulate genetic research, even though historically the black has revealed an incredible agility in sidestepping that plague's lethal scythe, a scythe which invariably chopped a wide swath through whites.

The disease is caused by a virus, injected into man by another kind of mosquito, the female Aëdes aegypti. The victim's first signal that all is not well is a general ill feeling; his features flush, he develops chills, then a high fever and quite possibly a severe head- and backache add to his distress. (Note that the symptoms to this point are sufficiently similar to a malarial attack to explain why the two were frequently confused.) Next the patient suffers through a restless, agitated two- or three-day period of high fevers and then characteristically experiences a remission. If he is fortunate, he has enjoyed (relatively speaking) a mild attack, and that is all there is to it.

If he is not fortunate, jaundice sets in a few hours later (hence “yellow” fever), the victim may hallucinate, internal hemorrhages occur, and he begins vomiting black blood (thus the Spanish appellation “vomito negro”) which “seems to gush forth without any effort from the patient.”