Dengue virus is back in the U.S. showing up in the Florida Keys according to a study released Tuesday by the Centers for Disease Control and the Florida Department of Health. The report details the exposure of about 5 percent or 1,000 Key West residents to dengue virus, followed by 12 confirmed cases of dengue in the area this year. This marks the first dengue outbreak in Florida since 1934.

“These people had not traveled outside of Florida, so we need to determine if these cases are an isolated occurrence or if dengue has once again become endemic in the continental United States,” said Harold Margolis, chief of the dengue branch of CDC in a press release.  “We are concerned that if dengue gains a foothold in Key West, it will travel to other Southern cities … like Miami.”

As the most common virus transmitted via mosquito, dengue causes up to 100 million infections, resulting in the death of up to 25,000 people every year. Dengue thrives in urban and semi-urban areas found in tropical and sub-tropical climates worldwide.

Symptoms include high fever, severe headache, severe pain behind the eyes, joint pain, muscle and bone pain. No vaccine exists that will prevent infection with dengue, but early detection and treatment prevents mortality. Locally acquired dengue outbreaks in the United States are rare.  The only reported instances of dengue outbreaks in the U.S. have been scattered along the Texas-Mexico border.