Frightening new forms of the MRSA superbug are emerging that attack the immune system, are highly infectious and can kill patients by destroying lung tissue.
The bacteria spread through the community and are not confined to hospitals like most MRSA strains.
They produce a toxin called Panton-Valentine leucocidin (PVL) that kills white blood cells, an essential part of the body's immune system defences.
Infection by one of the deadly strains could easily be overlooked because the initial symptoms appear relatively harmless.
By the time a correct diagnosis is made it might be too late to save the patient, a meeting of experts was told.
Dr Marina Morgan, from the Royal Devon & Exeter Foundation NHS Trust, said: "The new community associated MRSA strains appear to be more virulent and more easily spread between people.
"These community-associated versions have been found in people with few, if any, reasons to have MRSA. Typically they haven't recently been in hospital, or are not looking after or living with people with MRSA."
MRSA, methicillin-resistant Staphylococcus aureus, is a common bug carried by millions of people that has mutated to become immune to many antibiotics. If it gets into the bloodstream it can kill.
Already a major problem in hospitals, MRSA is now starting to spread to the wider population. Community MRSA is now well established in America, where it is a major cause of childhood infection.
Doctors are worried about the same pattern being repeated in Britain. The new strains appear to attach themselves to damaged skin and airways more easily than hospital MRSA, and they multiply at a faster rate. The warnings were spelled out at the Federation of Infection Societies Conference taking place at the University of Cardiff.
