Nearly 200 confirmed cases of the chikungunya virus in Puerto Rico and the first two cases of the mosquito-borne disease in Florida have been reported, say health authorities.
Puerto Rico's Health Department, in its weekly report, said that, as of June 25, laboratories had confirmed the virus in 182 people and that infections were suspected in 503 more.
There already have been dozens of deaths and thousands of people sickened by the virus in the Caribbean.
Symptoms include high fevers, joint and muscle pain and headaches. The virus is not transmissible between people, and is not usually fatal.
Of the cases confirmed in Puerto Rico, nearly a third (55) were recorded during the week of June 25, health officials there said.
Puerto Rico is a self-governing US territory.
Florida, meanwhile, reported its first two cases of people infected locally, one in Miami-Dade county and the other in Palm Beach county.
"The department has been conducting statewide monitoring for signs of any locally acquired cases of chikungunya," said Anna Likos, the Florida Health Department's director of disease control.
"We encourage everyone to take precautions against mosquitoes to prevent chikungunya and other mosquito-borne diseases by draining standing water, covering your skin with clothing and repellent and covering doors and windows with screens," she said.
The US Centers for Disease Control and Prevention announced the first case in Florida, an adult male, on Thursday.
Until now, the United States has only recorded cases of people coming back into the country with the virus after getting bitten while traveling, it said.
"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, chief of CDC's Arboviral Diseases Branch.
Since 2006, the United States has averaged 28 imported cases of chikungunya annually among people returning from countries in Asia, Africa, Europe, Central and South America and the Caribbean.
But this year it has recorded 357 cases.
There is no vaccine to prevent it and no medicine to treat it. However, the CDC says chikungunya is not often fatal and people usually recover within a week.
Symptoms are similar to dengue fever. Once a person is infected, he or she is likely to be protected against the disease in the future.