LONDON (Reuters) – Deadly pneumonia caused by so-called superbugs are spreading outside hospitals and represent a growing threat to the public, U.S. researchers warned on Wednesday.

Making the problem more worrying is the recent H1N1 flu outbreak because the "super-bug" pneumonia most commonly appears following an influenza-like illness, Alicia Hidron of the Emory University School of Medicine in Atlanta reported.

Methicillin-resistant Staphylococcus aureus (MRSA) infections can range from boils to more severe infections of the blood, lungs and the sites of surgery. Such infections can often be treated only with expensive intravenous antibiotics.

Most cases are associated with hospitals, nursing homes or other health care facilities but infections acquired in the wider community are increasing, the researcher noted.

"Community-acquired MRSA infections are no longer restricted to certain risk groups or to the geographic areas where outbreaks first occurred," they wrote in the journal Lancet Infectious Diseases.

"They now occur widely both in the community as well as health care facilities and have been reported on every continent."

As these superbugs spread in communities, more pneumonia cases are likely -- and the researchers said these may have mortality rates of more than 50 percent.

"However, the overall incidence of community-acquired MRSA pneumonia remains unknown," the researchers wrote.

In their analysis, Hidron and colleagues looked at two U.S. cases caused by a specific MRSA strain that is a culprit behind many superbug infections in the United States.

They reported the pneumonia usually features high fever and low blood pressure with rapid progression to septic shock -- a widespread infection that sends the whole body into a tailspin -- and an urgent need for mechanical ventilation.

The researchers said they do not know why community-acquired MRSA pneumonia appears so lethal but noted the bacteria that cause it are more susceptible to antibiotics than hospital-acquired infections.

"The best treatment of this ... disease has not been defined," they wrote.

(Reporting by Michael Kahn; Editing by Maggie Fox and Jon Hemming)