In the UK the superbug MRSA is associated with hospital infections, but doctors in America have discovered a super-resistant strain is plaguing gay men in San Francsico.

Known as USA300, the strain is carried by one in every 588 people in the city’s Castro district, thought to be the area with the highest proportion of gay men in the country.

Dr Binh Diep, from San Francisco General Hospital Medical Centre is one of the authors of the report, which was published online in the Annals of Internal Medicine.

“These multi-drug resistant infections often affect gay men at body sites in which skin-to-skin contact occurs during sexual activities,” he said.

“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.”

USA300 is a sub-type of Methicillin Resistant Staphylococcus Aureus (MRSA) and resitant to many anti-biotics.

The bacteria target tissue under the skin, causing large boils.

In some cases a flesh-eating form of pneumonia develops that attacks the lungs develops while other patients get blood poisoning.

The study was based on a review of medical records from outpatient clinics in nine of the ten medical centres in San Francisco and some other clinics in Boston.

It found that sexually active gay men in San Francisco are 13 times more likely to be infected with USA300 than the general population.

So far two cases have been confirmed in the UK, according to Channel 4 News.