Health authorities have detected the emergence of a rare but deadly "lung-eating" form of pneumonia sparked by the combination of a skin infection and the common flu.
The national Centers for Disease Control and Prevention reported 22 deaths among children in 2007 from the dual infection.
Numbers from the 2007-08 flu season won't be released until October, but officials say that deaths have increased. The CDC has just begun tracking cases among all age groups.
The number of fatalities, while low, is a sharp increase from previous years, and infectious disease experts worry that an ongoing epidemic of skin infections could drive the numbers higher.
The double infection has appeared before: It was the leading cause of bacterial pneumonia deaths during the 1957-58 flu pandemic, which killed 2 million people worldwide and about 70,000 people in the United States.
This time, health authorities are putting out a call for people to get a simple annual flu vaccine to protect themselves.
"Since so many of these pneumonias are associated with influenza, the best prevention is to prevent influenza," said Jeffrey C. Hageman, a CDC epidemiologist.
The main culprit is a particular strain of the bacterium Staphylococcus aureus. About 1 in 3 people carry some variety of Staphylococcus in their noses or skin, usually without harm.
An antibiotic-resistant strain known as "methicillin-resistant Staphylococcus aureus," or MRSA, has been a known killer since it emerged in hospitals in the 1960s, preying on elderly and frail patients.
But in the last decade, a new, more virulent strain has emerged outside of hospitals, causing an explosion in severe skin infections.
The infections can be spread by skin-to-skin contact, by sharing towels, razors and other personal items or from infected surfaces. Outbreaks are common among wrestlers, football players and others who play contact sports.
Many of the skin infections heal after being opened and drained of pus. In a few cases, the bacteria can cause severe invasive infections of bones, joints, blood and the lungs.
The community strain is not as resistant to antibiotics as the hospital strain and many drugs are still effective. What makes it so lethal is the toxins it produces.
A few years ago, health authorities noticed that the community strain of the virus and the common flu seemed to be teaming up to create a dangerous confluence of infection.
In 2003-2004, the CDC found five fatal cases of Staphylococcus pneumonia and flu among previously healthy children and adults. It asked states to report all flu deaths in children. In 2006-2007, the number jumped to 22.
Flu makes it easier for bacteria in the nose or throat to reach the lungs by stripping the respiratory tract of the coating that filters out bacteria. It also may keep the immune system too tied up to respond to a second invasion.
The result, scientists believe, can be an aggressive necrotizing pneumonia that destroys lung tissue. Survival often requires immediate surgery to drain lungs of pus and fluids, then weeks in the hospital on intravenous antibiotics. The fatality rate has been estimated from 13 percent to as high as 50 percent.
"There's a big fear right now, and there are some good indications, that the current Staphylococcus strain works very well with influenza," said Dr. Jonathan A. McCullers, an infectious disease specialist at St. Jude's Children's Hospital in Memphis, Tenn.
Scientists have seen no sign that MRSA pneumonia itself is exceptionally contagious. When individual children have contracted it, no outbreaks in schools or among other groups followed.
"People don't need to go out and start wearing space suits," said Dr. Brad Spellberg, an infectious diseases expert at the University of California, Los Angeles. "But we need to get smarter about how we deal with it."
State and national health officials are trying to get a clearer sense of how frequently co-infections occur among all ages and what proportion is fatal.
In February, California made severe staph infections in a previously healthy person resulting in death or admission to an intensive care unit immediately reportable to local health departments.
Just before that, the CDC issued a health advisory, alerting family and emergency physicians to be on the watch for flu and Staphylococcus infections even in young, otherwise healthy patients.
The CDC also expanded its recommendations on who should be vaccinated for influenza to include children up to age 19. While anyone over 6 months of age can get a flu shot, priority previously was given to children up to age 4, adults over age 50, pregnant women and those with chronic illnesses or immune disorders.
Besides a flu shot, Dr. Lynn Finelli, head of influenza surveillance for the CDC, said frequent hand washing and other basic hygiene can help.
"We think that good hygiene can both prevent people from being colonized and keep the bug from taking hold of you," she said.