Viruses of group C and group Guama were among the first agents isolated in 1954 when researchers of the Special Service of Public Health of Brazil and the Rockefeller Foundation set out to find the causes of human fevers at Belem, Brazil, near the mouth of the Amazon River. Persons infected by group C and group Guama viruses develop fever of sudden onset ranging from 38 to 40°C. Patients with group Guama virus infections usually have lower grade fevers than those with group C infections. Severe headache is the dominant complaint. Group C and group Guama viruses are found only in the American tropics and subtropics, including Florida, Central America, northern South America, and southern Brazil. There is no reason to believe that susceptibility varies with sex or race. The group C and Guama fever cases which have been recorded have been predominantly in young adult males.