Health officials say 5 percent of Key West residents show evidence that they have been exposed to the virus that causes dengue fever.

The federal Centers for Disease Control and Prevention and the Florida Health Department said Tuesday they launched the study of antibodies after three dengue cases surfaced in Key West last August, the first locally acquired cases reported in the state in four decades.

Officials say 39 residents and visitors have actually come down with the mosquito-borne disease.

Dengue fever is a flu-like illness spread by the bite of an type of infected mosquito that is common urban mosquito in the U.S. and Caribbean.

It is also known as breakbone fever because of the intense joint pain suffered in extreme cases.