WASHINGTON - A dangerous germ that has been spreading around the country is killing more people in the United States each year than the AIDS virus, federal health officials reported Tuesday.

The microbe, a strain of a once-innocuous staph bacterium that has become invulnerable to first-line antibiotics, is responsible for more than 94,000 serious infections and nearly 19,000 deaths each year, the Centers for Disease Control and Prevention calculated.

Although evidence has been mounting that the infection is becoming more common, the estimate published today in the Journal of the American Medical Association marks the first national assessment of the toll from the insidious pathogen, officials said.

"This is the first study that's been able to capture the data in a comprehensive fashion," said Scott K. Fridkin, a medical epidemiologist at the CDC. "This is a significant public health problem. We should be very worried."

Other researchers noted that the estimate includes only the most serious infections caused by the bug, known as methicillin-resistant Staphylococcus aureus, or MRSA.

"It's really just the tip of the iceberg," said Elizabeth Bancroft, a medical epidemiologist at the Los Angeles Department of Public Health who wrote an editorial accompanying the new research. "It is astounding."

On Monday, a Lynch Station, Va., teenager, Ashton Bonds, 17, succumbed to MRSA, prompting a shutdown of 21 Bedford County schools for cleaning to prevent further infections. The infection had spread to Bonds' kidneys, liver, lungs and the muscle around his heart.

A second report in the same journal said that a strain of another bacterium, which causes ear infections in children, has become impervious to every approved antibiotic for youngsters.

"Taken together, what these two papers show is that we're increasingly facing antibiotic-resistant forms of these very common organisms," Bancroft said.

The reports underscore the need to develop new antibiotics and curb the unnecessary use of those already available, experts said. They should also alert doctors to be on the lookout for antibiotic-resistant infections so patients can be treated with the few remaining effective drugs before they develop serious complications, experts said.

MRSA is a strain of the ubiquitous bacterium that usually causes "staph" infections that are easily treated with common antibiotics in the penicillin family, such as methicillin and amoxicillin. Resistant strains of the organism, however, have been increasingly turning up in hospitals and in small outbreaks outside of heath-care settings, such as among athletes, prison inmates and children.

The germ, which is spread by casual contact, rapidly turns minor abscesses and other skin infections into serious health problems, including painful, disfiguring "necrotizing" abscesses that eat away tissue. The infections can often still be treated by lancing and draining sores and quickly administering other antibiotics, such as bactrim. But in some cases the microbe gets into the lungs, causing unusually serious pneumonia, or spreads into bone, vital organs, and the bloodstream, triggering life-threatening complications. Those patients must be hospitalized and given intensive care, including intravenous antibiotics such as vancomycin.

Fridkin, the CDC epidemiologist, said the new data show that "life-threatening MRSA infection is much more common than we had thought."

In fact, the estimates make MRSA much more common than flesh-eating strep infections, bacterial pneumonia and meningitis combined, Bancroft noted.

"These are some of the most dreaded invasive bacterial diseases out there," Bancroft said. "This is clearly a very big deal."

The infection is most common among African-Americans and the elderly but also commonly strikes very young children.

"We see these cases all the time," said Robert Daum of the University of Chicago. "In the last five weeks I've taken care of five children who were sick enough to be hospitalized and require intensive care."

Studies have shown that hospitals could do more to reduce the spread of the infection through standard hygiene measures. Individuals can reduce their risk through common-sense measures, such as frequent hand-washing.