MRSA is usually a threat in healthcare settings
A new strain of MRSA seems to be triggering a deadly form of pneumonia in people who catch flu, experts say.
Researchers believe the new strain of the antibiotic-resistant bacterium is becoming more widespread.
It is known as community acquired MRSA, (CA-MRSA) because, unlike most forms of the superbug, it poses a significant risk outside hospitals.
The potential threat is detailed in a study appearing in the journal Lancet Infectious Diseases.
However, experts stressed cases of pneumonia caused by CA-MRSA in the UK were very rare.
The researchers, from Emory University School of Medicine in Atlanta, say death rates following infection may be higher than 50%.
They warn the emergence of swine flu could intensify the problem, as CA-MRSA appears to strike people who are already ill with flu.
However, the just how common infections are remains unknown.
MRSA infections can range from boils to more severe infections of the blood, lungs and sites of surgery.
Most occur in healthcare settings, such as hospitals or nursing homes.
But the latest study warns that community-acquired cases are beginning to increase.
Writing in the journal, the researchers said: "Community-acquired MRSA infections are no longer restricted to certain risk groups or to the geographic areas where outbreaks first occurred."
"They now occur widely both in the community as well as health care facilities and have been reported on every continent."
The latest study is based on an analysis of two cases from the US.
In both, the patients developed bacterial pneumonia, high fever and low blood pressure, and rapidly progressed to septic shock, a widespread infection that requires urgent medical attention.
They both made a full recovery.
The reason why the bacterium appears so lethal remains unclear.
It was thought to have emerged from Australia in the 1990s, but initially only triggered relatively trivial skin and soft tissue problems.
However, antibiotics do appear to have more effect than in standard MRSA infections.
Professor Mark Enright, an expert in MRSA at Imperial College London, said: "Bacterial pneumonia following influenza can be very serious and in some cases fatal.
"CA-MRSA pneumonia is particularly dangerous due to the rapid, aggressive nature of the infection and the difficulty in providing effective chemotherapy.
"The emergence of pandemic influenza and increased prevalence of CA-MRSA in many countries may cause increased morbidity and mortality in infected individuals."
Professor Ron Cutler, of Queen Mary University Of London, said: "In the past respiratory tract infections with MRSA tended to be in the elderly in a hospital ward.
"These infections could have been post viral and with a weakened immune system and a poor response to antibiotics were and still are difficult to treat successfully.
"The CA strains are able because of their increased toxic potential to infect a younger population."
Professor Richard James, of the University of Nottingham, said: "The threat from CA-MRSA in the USA is very serious concern, especially if there is a flu epidemic as this could trigger a large number of cases of necrotizing pneumonia, which has a mortality rate of more than 50% in 72 hours.
"The concern is that this may be the start of an exponential increase as we saw with hospital MRSA infections in the 1990s.
"It took the UK 13 years to get to grips with hospital-acquired MRSA infections, we are not equipped to deal with large numbers of CA-MRSA infections in the community."
However, the Health Protection Agency stressed that in the UK only a tiny fraction of Staphylococcus aureus bacteria carried the relevant disease-causing toxin, know as PVL.
And of these, most were more likely to cause minor infections, rather than pneumonia.
Dr Angela Kearnes, head of the HPA's Staphylococcus aureus reference lab, said: "Although several other countries have encountered serious problems, it is important to note that these infections remain uncommon in the UK."