Health authorities have detected the emergence of a rare but deadly lung-destroying 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 last year from the dual infection.
Numbers from the 2007-2008 flu season won't be released until next month, but officials say deaths have increased. The CDC has just begun tracking cases among all age groups.
The number of fatalities, though 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 1957-1958 flu pandemic, which killed 2 million people worldwide, including about 70,000 in the U.S.
This time, health authorities are putting out a call for people to get an 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 strain of the bacterium Staphylococcus aureus. About 1 in 3 people carries some variety of Staphylococcus in the nose 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 the elderly and frail.
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 and other personal items; or from infected surfaces. Outbreaks are common among people who play contact sports.
Most 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 lungs.
The community strain is not as resistant to antibiotics as the hospital strain. What makes it so lethal is the toxins it produces.
Health authorities began to notice a few years ago that the community strain and the common flu seemed to be teaming up to create a dangerous confluence of infection.
In the 2003-2004 flu season, the CDC noticed five fatal cases of Staphylococcus pneumonia and flu among previously healthy children and adults. It began asking states to report all flu deaths in children. In the 2006-2007 season, 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 helps filter 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 surgery to drain lungs of pus and fluids, then weeks in the hospital on antibiotics. Various estimates put the fatality rate at 13% to 50%.
"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 Research Hospital in Memphis, Tenn.
Scientists have seen no sign that pneumonia caused by the community strain is exceptionally contagious. When individual children have contracted it, no outbreaks followed in schools or among other groups.
"People don't need to go out and start wearing spacesuits," said Dr. Brad Spellberg, a UCLA infectious disease expert. "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 and how often they are fatal.
In February, California began requiring that severe staph infections -- those in previously healthy people that resulted in death or intensive-care admission -- be immediately reported to local health departments.
Just before that mandate was announced, the CDC told family and emergency physicians to be on the watch for flu and staph infections.
The CDC also expanded its recommendations on flu vaccinations for youths to include anyone 6 months to 18 years old. (Previously, vaccination was not recommended for most people age 5 to 18.) It continues to recommend flu shots for people who are 50 or older, pregnant, or have chronic illnesses or immune disorders.
Besides a flu shot, frequent hand-washing and other basic hygiene measures can help, said Dr. Lyn Finelli, head of influenza surveillance for the CDC. "We think that good hygiene can both prevent people from being colonized and keep the bug from taking hold of you," she said.