WEST PALM BEACH, Fla. — Health officials are reporting that for the first time, U.S. mosquitoes are spreading a virus that has been tearing through the Caribbean.

Two people in Florida have domestically acquired chikungunya infections, officials said Thursday. In both cases, they said, a person infected with the virus after visiting the Caribbean was then bitten again by an uninfected mosquito in Florida, which then transmitted the illness further.

Health officials urged residents to prevent mosquito bites, but said there was no cause for alarm.

“There is no broad risk to the health of the general public,” said Dr. Celeste Philip, a public-health official with the Department of Health.

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Federal officials noted it’s an unfortunate milestone in the spread of a painful infectious disease that has raced across the Caribbean this year and is apparently now taking root in the continental United States.

“The arrival of chikungunya virus, first in the tropical Americas and now in the United States, underscores the risks posed by this and other exotic pathogens,” said Roger Nasci of the Centers for Disease Control and Prevention (CDC).

Health officials in Puerto Rico, meanwhile, on Thursday declared an epidemic of the virus in the U.S. territory, which reported its first case in late May.

Health Secretary Ana Rius said more than 200 cases had been confirmed on the island as of June 25 and that the majority of them were reported in the capital of San Juan and nearby areas.

Chikungunya virus is rarely fatal. Infected people typically experience fever, severe joint pain and swelling, muscle aches, headaches, or rash. Patients usually recover in about a week, although some people suffer long-term joint pain. There is no vaccine and no specific treatment.

This virus is not spread person to person, but rather by the bite of certain mosquitoes. That’s why health officials believe the virus is spreading in the continental U.S.; the two cases had not recently left the country.

The infected Floridians were described as a 41-year-old woman in Miami-Dade County who began experiencing symptoms June 10, and a 50-year-old man in Palm Beach County, who first noticed symptoms July 1.

Philip said both are doing well.

State epidemiologist Anna Likos said that for the virus to be transmitted from an infected person to an uninfected mosquito, they must be bitten within the first week of illness.

Chikungunya was first identified in 1952 in Tanzania. It first appeared in the Americas late last year, on a Caribbean island. By July 11, more than 355,000 suspected and confirmed cases were being reported in the Americas.