Sometimes the lungs, weakened by the flu, become prey to another infection, often streptococcus, and the body is felled by bacterial overload, as happened to a New Hampshire mother of four earlier this month.
Worldwide, the flu causes up to 640,000 deaths annually.
Doctors have long known that contracting influenza can be dangerous for the elderly, for infants and for those already in a weakened state. But flu can kill others as well, depending on the virulence of the particular strain that spreads during flu season.
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This year's strain is the most severe in a decade.
A chart from Children's Healthcare of Atlanta can help parents determine when to seek help.