Yellow fever is a highly fatal, incurable viral infection. Not all who are bitten by an infected mosquito develop symptoms but many of those who do experience fever, headache, pain, nausea and dizziness over a period of three to six days at which point they recover and are thereafter fully immune. Less fortunate are those who develop the classic symptoms of yellow fever. Following a brief period of remission, the onset of classic yellow fever is signaled by the return of high fever together with nausea and vomiting. It is at this point that the more frightening symptoms of jaundice, kidney failure and hemorrhage appear, leading at times to the black vomit or vomito negro for which the condition was once known. As an incurable infection, physicians can do very little for people suffering from yellow fe ver and half of those who develop severe symptoms experience shock, seizures and become comatose before dying. This high mortality and lack of effective treatment make the return of yellow fever a fearful prospect and explain why it was once a terrifying disease. In the Americas, yellow fever was, for a period of several hundred years, an unrelenting source of epidemics causing significant loss of life and endless commercial disruption.