As a local high school football field was tested this week and found clear of a drug-resistant strain of bacteria, health experts noted that serious infections caused by the pathogen, known as MRSA, are much more likely among people who have had recent treatment in health care settings such as hospitals and nursing homes.

A study released earlier this week suggests that most invasive, severe disease from MRSA infection "is associated with health care delivery and occurs in people with underlying illness," said Dr. John Jernigan, a deputy chief in the division of health care quality promotion for the Centers for Disease Control and Prevention.

The study's publication in the Journal of the American Medical Association coincided with news accounts of MRSA infections in schools around the nation, often among student athletes.

The study found that invasive, severe disease can result from those or other types of MRSA infections acquired in the community, without prior association with the health care system. But those cases are "relatively rare," Dr. Jernigan said.

"Do we think people should panic? Absolutely not," said Michael Huff, deputy secretary of health planning and assessment for the Pennsylvania Department of Health.

Dr. Andrew Sahud, chairman of the infection control and prevention committee at Allegheny General Hospital, noted that antibiotics and certain preventive measures, such as hand-washing, keeping cuts and scrapes covered, and not sharing towels or other personal items, are often effective against MRSA.

The Allegheny County Health Department agreed to conduct the turf tests at Mt. Lebanon High School after a meeting with parents. Officials said concerns were raised that staph germs might be in the turf.

Ten cases of methicillin-resistant Staphylococcus aureus, or MRSA, have been confirmed at the high school, nine of them involving football players and the 10th a student athlete from another team. Three more students with suspected cases are being tested and one case has been confirmed at Jefferson Middle School.

A total of 13 specimens, eight from the turf and five from indoor facilities, tested negative for staph bacteria, Health Department spokesman Guillermo Cole said yesterday.

"The fact that all of the results came back as rapidly as they did is a testament to how clean things are," Mr. Cole said. "They support what we believed all along -- that the field is an unlikely source of staph."

Research last year by Penn State soil scientists had similar results. No staph was found on the playing fields, but it was found on weight equipment, a table, used towels and blocking pads. It was also found on every hand sample taken at the sites.

The study showed "these fields are not 'incubators' for staph as some were claiming," said Andrew S. McNitt, the study's lead author and a professor of soil science-turfgrass. "We know it can survive on inanimate objects for a limited amount of time."

The MRSA study published this week suggests the pathogen may have been associated with more than 94,000 serious infections nationwide in 2005 and nearly 19,000 deaths.

The study focused on invasive MRSA infections -- those that spread to normally sterile areas such as the blood, joints or organs. Based on data collected from nine sites, researchers estimated that the infections were linked to more deaths in 2005 than those associated with the AIDS virus.

The study said 85 percent of invasive infections were associated with health care treatment, such as a recent stay in a hospital or nursing home. Those without documented health care risk factors -- nearly all the other cases -- were considered community-associated cases.

Researchers said their findings further document that invasive MRSA disease occurs in community-related cases and can involve "significant mortality."

Earlier this week, a high school student in Virginia died after being hospitalized for a MRSA infection.

The study noted, however, that death rates were lower for community-associated MRSA infections than for those linked to health care settings.

Researchers also observed that noninvasive, community-related MRSA infections "greatly outnumber" invasive cases and said MRSA cases linked to the community usually involved skin disease. MRSA has become the most common cause of skin and soft tissue infections in U.S. emergency departments.

A Wilkins dermatologist, Dr. Lisa Pawelski, said she has noticed a dramatic increase in MRSA infections. Her practice sees two or three cases a month.

At the county health department, Mr. Cole said although pediatricians are the only health professionals required to report MRSA cases, physicians throughout the county are encouraged to report cases.

Dr. Jernigan noted that among the invasive, community-associated MRSA cases in the study, only about 20 percent arose from people with skin infections. Others came from people with problems such as pneumonia or bone infections.

In the Pittsburgh area, some hospitals have worked for years to reduce MRSA infections.

"A number of facilities in southwestern Pennsylvania are leading the curve in health-care associated MRSA prevention," said Dr. Jernigan.

Hospitals can use nasal swabs to test for the pathogen, then isolate patients who test positive.

Pennsylvania lawmakers also approved legislation earlier this year aimed at reducing health care-acquired infections from MRSA and other sources. The legislation was modified from a proposal made by Gov. Ed Rendell as part of his "Prescription for Pennsylvania" health plan.

By mid-February, hospitals will begin reporting those infections to a data system operated by the CDC, said Stacy Mitchell, acting deputy secretary for quality assurance for the state health department. Nursing homes will begin reporting similar data later next year, she said.

State health officials will review the data for trends and use it to help facilities improve their infection rates, she said.