Chikungunya causes fever, severe joint pain and swelling, muscle aches, headaches or rashes and can be fatal. Institut Pasteur

A virus previously only known in the Caribbean has been transmitted in the US for the first time ever.

The painful mosquito-born Chikungunya virus passes solely via the insect – not person-to-person.

Two cases of infection have been reported, both in Florida. The patients are a 41-year-old woman near Miami who began experiencing symptoms on June 10 and a 50-year-old man near Palm Beach, who first noticed the illness on July 1.

In neither case had the infected person travelled out of the country just before becoming infected, leading US medical experts to believe some American mosquitos now carry the virus.

Chikungunya virus is incurable and can be fatal. Infected people usually suffer from fever, severe joint pain and swelling, muscle aches, headaches or rashes. The illness is reportedly raging in the Caribbean, where all prior case have been reported.

"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), in a statement.

There were more than 230 Chikungunya cases reported in Americans this year, but all the others were travellers thought to have been infected elsewhere.

Both new patients are said to be doing well.