More than half of the skin infections treated in US emergency rooms are caused by bacterial infections resistant to treatment, a new study suggests.

Although rates varied considerably among the hospitals, MRSA was blamed for 59 percent of skin infections overall in the facilities involved in the study, reported a team of investigators from UCLA and the CDC.

Most of the time, MRSA is harmless, but on rare occasions, it can be serious -- even fatal. MRSA infection often manifests as a painful boil or pimple, usually swollen and red.

Commonly misidentified as spider bites, MRSA infections can cause sores, lesions, and pneumonia. It used to show up primarily among hospital and nursing home patients, but is now a common bacteria that is estimated to be present in about a third of the US population at any given time.

In the recent hospital study, 99 percent of the patients treated for skin infections had acquired MRSA outside the hospital. Almost all of those cases were caused by the USA300 strain that is now prevalent across the US.

Physicians should take drug resistance into account when treating skin infections, urges Rachel Gorwitz of the CDC, co-author of the study. She advises doctors to take cultures from skin infections to identify the strain of bacteria responsible.

In 57 percent of the cases followed by the researchers, infected patients were prescribed the wrong drugs -- that is, medications the bacteria were resistant to -- even though there are several drugs available that are effective in treating MRSA.