Drug-resistant bacteria are infecting more people in community settings such as prisons and public housing, and not just in hospitals where such "superbugs" can run rampant, researchers said on Monday.

Over a five-year period, researchers at a Chicago hospital found a seven-fold increase in drug-resistant staph infections that had been contracted outside of any hospital.

They projected the rate of infection rose to 164 cases per 100,000 people in 2005, up from 24 cases per 100,000 in 2000.

The stubborn infections -- known as methicillin-resistant Staphylococcus aureus -- do not respond to standard antibiotic treatment, said the report published in the Archives of Internal Medicine.

Generally, such infections have been confined to vulnerable patients in hospitals. But over the past decade the problem has emerged in community settings around the world.

After analyzing 518 people who received treatment for community-contracted infections, researcher Bala Hota of Chicago's Rush University Medical Center pointed to two risk factors: incarceration in a jail or prison, and living in public housing projects.

Other risk factors linked to so-called community-contracted infections include intravenous drug use, living in overcrowded housing, playing certain sports, tattooing and poor hygiene, the report said.

"An understanding of factors promoting acquisition and emergence of (drug-resistant infections) may aid in the development of prevention strategies," Hota wrote.

Drug-resistant infections caught in U.S. hospitals kill 90,000 people annually and cost $4.5 billion, according to the U.S. Centers for Disease Control and Prevention. The problem has also been documented in Canada and Britain.

Most scientists have blamed overuse of antibiotics for creating drug-resistant bacteria, which evolve to evade treatment. Hospital workers who neglect simple hand-washing can spread the infection.