Experts said they are remaining vigilant after seeing 100 cases of a superbug in healthy people in England and Wales over the last three years.

MRSA is commonly thought of as a hospital superbug.

But a strain has been identified which causes skin infections in healthy people living in the community.

The Health Protection Agency said one 28-year-old woman had died from pneumonia after contracting the condition.

It's difficult to know how common it actually is because of the lack of a proper surveillance system. Dr Donald Morrison, Scottish MRSA Reference Laboratory

The severe form of pneumonia is a rare complication of CA-MRSA.

MRSA (methicillin-resistant Staphylococcus aureus) is a recognised problem in hospitals and nursing homes, where those infected are weakened by illness.

However, experts have identified the genetically different strain of the disease which is affecting healthy people in the community.

This form of MRSA appears as sores which look like insect bites, and is spread by skin-to-skin contact. If untreated, it can develop into abscesses and boils.

It is resistant to methicillin, but does respond to several other antibiotics.

High risk groups

In the US, the condition has been linked to health-clubs and gyms. But the HPA said there had been no reports of cases being picked up in that way in this country.

Instead, it said the risk would be higher in contact sports such as wrestling and rugby, where open wounds could become infected using dirty towels.

Other groups at high risk include prisoners and intravenous drug users.

Around 30% of the population carry Staphylococcus aureus in their noses or on their skin, but are not made ill by it.

The 100 cases reported to the HPA were predominantly those who were infected with the strain, although some were simply people who carried CA-MRSA.

Dr Donald Morrison, of the Scottish MRSA Reference Laboratory, told the BBC News Website: "These numbers are an indication of the prevalence of community-acquired MRSA.

"But it's difficult to know how common it actually is because of the lack of a proper surveillance system."

Liberal Democrat health spokesman Paul Burstow said: "These new findings must mean that community infections should now register on the government's radar."

Death certificates

In a separate development on Monday, hospitals in England were told they should be kept clean "all day, every day" on 'Think Clean Day', thr latest drive to improve hygiene and battle the superbugs.

It came as health workers union Unison said that employing more hospital cleaners should be the main weapon in the battle against deadly superbugs such as MRSA.

Shadow Health Secretary Andrew Lansley said: "Every day should be think clean day. We need consistent, not piecemeal action."

Last week figures from the Office for National Statistics (ONS) showed that the number of deaths in which MRSA was a factor doubled in four years.

The number of death certificates mentioning MRSA rose from 487 in 1999 to 955 in 2003.