A deadly strain of the superbug MRSA which can lead to a flesh-eating form of pneumonia has emerged.
Research suggests it may be more prevalent among the gay community - the gay San Francisco district of Castro appears to have been hardest hit.
So far only two cases of the new form of the USA300 strain of the bug have been recorded in the UK.It is not usually contracted in hospitals, but in the community - often by casual contact.The new strain is resistant to treatment by many front-line antibiotics.
It causes large boils on the skin, and in severe cases can lead to fatal blood poisoning or necrotising pneumonia, which eats away at the lungs. Researchers say the bug has so far been 13 times more prevalent in gay men in San Francisco than in other people.
In the Castro district - where more gay people live than anywhere else in the US - about one in 588 people are carrying the bug. In the general San Francisco community the figure was around one in 3,800.
Researcher Dr Binh Diep, from San Francisco General Hospital Medical Centre, said: "These multi-drug resistant infections often affect gay men at body sites in which skin-to-skin contact occurs during sexual activities.
"But because the bacteria can be spread by more casual contact, we are also very concerned about a potential spread of this strain into the general population."
Dr Diep said the best way to avoid transmission was probably to wash thoroughly with soap and water, especially after sexual activities.
The study, reported in Annals of Internal Medicine, was based on a review of medical records from outpatient clinics and medical centres in San Francisco and Boston. Professor Mark Enright, from Imperial College and St Mary's Hospital, London, Britain's leading authority on MRSA, said: "It's quite surprising that the figures are so high. "We do know that the USA300 strain is extremely good at spreading between people through skin-to-skin contact.
"The main reservoir for this infection is gay men, drug users, and those involved in contact sports, like wrestling. Having lots of sexual partners and making skin contact with a large number of different people helps the infection to spread.
"In the US it is already moving into the wider community, which is very worrying."