Cases of antibiotic-resistant staph infections usually acquired in the community are growing in the United States and rapidly spreading to hospitals, researchers said Tuesday.

Methicillin-resistant Staphylococcus aureus (MRSA), which can penetrate wounds and cause lethal bloodstream and lung infections, kills about 20,000 each year in the United States, according to the Centers for Disease Control and Prevention (CDC).

That is more deaths than the over 14,000 people who died from AIDS in the US in 2007.

Although the superbug has usually spread in hospitals, "the findings from this study suggest that there is a significant reservoir in the community as well," said lead author Eili Klein, researcher at Resources for the Future.

The study analyzed data from over 300 microbiology labs serving hospitals across the country, finding a seven-fold jump in the proportion of community-associated strains of MRSA at outpatient hospital units between 1999 and 2006.

This increase threatens the safety of patients because health care professionals and patients travel frequently between a hospital's inpatient and outpatient units, the authors warned.

More than 63,000 people die in the United States each year from hospital-acquired infections resistant to at least one antibiotic.

Although they are less virulent and susceptible to more antibiotics, community-associated strains can still cause significant illness and death, according to the study published in the December issue of "Emerging Infectious Diseases."

Those strains can be acquired in virtually any public place.