http://www.cdc.gov/mmwr/PDF/wk/mm4625.pdf

On October 15, 1996, a physician notified the Illinois Department of Public Health about five patients with an unknown febrile illness who had returned from a white-water rafting trip on flooded rivers in Costa Rica during September 27-28, 1996. This report summarizes the findings of the multistate investigation conducted by the Illinois Department of Public Health and by CDC in collaboration with the Ministry of Health of Costa Rica. The findings implicated leptospirosis as the cause of disease and contaminated river water as the probable source of illness.