(Reuters) - Five percent of the population of Key West, Florida -- more than 1,000 people -- have been infected at some point with the dengue virus, government researchers reported on Tuesday.

Most probably did not even know it, but the findings show the sometimes deadly infection is making its way north into the United States, the researchers said.

"We're concerned that if dengue gains a foothold in Key West, it will travel to other southern cities where the mosquito that transmits dengue is present, like Miami," said Harold Margolis, chief of the dengue branch at the U.S. Centers for Disease Control and Prevention.

"These cases represent the reemergence of dengue fever in Florida and elsewhere in the United States after 75 years," Margolis said in a statement.

"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."

Dengue is the most common virus transmitted by mosquitoes, infecting 50 million to 100 million people every year and killing 25,000 of them.

It can cause classic flu-like symptoms but can also take on a hemorrhagic form that causes internal and external bleeding and sudden death. Companies are working on a vaccine but there is not any effective drug to treat it.

Dengue was eradicated in the United States in the 1940s but a few locally acquired U.S. cases have been confirmed along the Texas-Mexico border since the 1980s. More cases have been reported recently in Mexico and the Caribbean.

After 27 cases of dengue were reported in Florida in 2009, scientists from the CDC and the Florida Department of Health took blood samples from 240 randomly chosen Key West residents.

Of these, 5 percent had active dengue infections or antibodies to the virus, showing they had been infected, the researchers told the International Conference on Emerging Infectious Diseases being held in Atlanta.