ATLANTA — Darriel Fleming thought he had a spider bite.

As the boil on his stomach swelled painfully in July 2007, the 28-year-old Marietta, Ga., man went to a nearby emergency room. A doctor drained the wound and sent Fleming home with a prescription for antibiotics.

Cox News Service photo (ENLARGE)

Microbiologist Sigrid McAllister holds a sample of colonies of Staphylococcus aureus in the National Reference Laboratory for Staphylococcus at the federal Centers for Disease Control and Prevention.

Three weeks later, Fleming was dead, killed by a drug-resistant form of staph bacteria called MRSA.

MRSA, or methicillin-resistant Staphylococcus aureus, for decades was an infection confined to hospitals and nursing homes. But its strains increasingly are infecting people in the community, outside of health-care settings.

Local and federal health officials are working to educate doctors and the public about the rise in these drug-resistant infections – best evaluated and treated early – and on ways people can protect themselves. They emphasize that the vast majority of community MRSA infections are treatable skin boils and abscesses.

Sometimes, for reasons unclear to scientists, the bacteria invade a person's bloodstream or wreak havoc with internal organs.

Researchers are studying who gets these serious community MRSA infections and examining the bacteria for clues about why they live on some people without causing infection – yet kill others.

"We want people to understand that potentially, even though it's very uncommon, these community MRSA cases can be very serious and even fatal," said Dr. Rachel Gorwitz, epidemiologist at the federal Centers for Disease Control and Prevention in Atlanta.

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Staph bacteria, even those that aren't resistant to antibiotics, have long caused serious infections.

In the 1960s, the first reports surfaced of staph infections that had stopped responding to the antibiotic methicillin. Over the decades, those strains have spread, and the germs have developed resistance to other drugs, largely in hospitals where they infect patients weakened by disease or made vulnerable through surgical wounds and catheters.

Even today, MRSA poses the greatest threat in hospitals, where strains are usually genetically different and, because of antibiotic use, more difficult to kill than those circulating out in the community. About 85 percent of life-threatening, invasive MRSA infections involve people who have been hospitalized, lived in a nursing home or been treated in some other health-care facility, the CDC estimates.

During the 1980s, doctors began finding cases of MRSA in people who hadn't spent time in health-care settings.

Unlike hospital strains, MRSA in the community tended to cause skin infections.

Unlike its hospital cousins, community MRSA still responds to a wider range of antibiotics, experts said. It's unusual for community infections to become life-threatening.

Still, MRSA made headlines last fall – and caused the panicked closure of some schools across the nation – in the wake of publicity about a new study estimating that more life-threatening infections occur than previously thought.

The study, which involved CDC and Emory University researchers, estimated invasive MRSA infections – in health-care facilities and the community – killed nearly 19,000 Americans in 2005. Another 94,000 had life-threatening infections.

About the same time, news reports played up the MRSA death of a Virginia teen. As public awareness and concern grew, schools elsewhere sent home notes alerting parents when a child had a routine, treatable MRSA boil. Some schools closed for disinfection.

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Lost in the furor was the study's main finding: Most life-threatening MRSA infections – 85 percent – involve people who have been infected while hospitalized or living in a nursing home. Only about 15 percent happened in the community.

People 65 and older were most likely to suffer invasive MRSA infections of all types, the study found. Blacks had invasive MRSA at nearly twice the rate of whites, which researchers speculate could be due to higher rates of chronic illnesses that may make them more vulnerable.

At the CDC's campus in Atlanta, scientists in the national Staphylococcus reference laboratory examine samples of MRSA from people such as Darriel Fleming.

They grow the bacteria in large incubators, then determine what antibiotics will and won't kill them. They also run tests to identify the bacteria's genetic fingerprints and whether they are producing certain toxins.

A lot remains unknown about MRSA – especially why it lives on some people without infecting them, and why in some cases it kills.

"That's really an important question for us to learn more about," said Gorwitz, the CDC epidemiologist.

It may involve differences within MRSA strains, a person's genetics or delays in seeking care, she said.

Until scientists have more answers, experts said, the public's best protection is to practice good hygiene and get skin problems checked out by a doctor.

Cox News Service researcher Sharon Gaus contributed to this report.