Influenza viruses circulate widely in the United States usually beginning in late fall and continuing until early spring, although the onset, peak and season end cannot be predicted accurately. The severity of influenza illness depends on a number of factors related to both the host and to the virus.

Image courtesy of the Centers for Disease Control and Prevention

This electron micrograph shows recreated 1918 influenza virions. The 1918 Spanish flu epidemic was caused by an influenza A (H1N1) virus, killing more than 500,000 people in the United States, and up to 50 million worldwide. The possible source was a newly emerged virus from a swine or an avian host of a mutated H1N1 virus. Nearly half of those who died were young, healthy adults.

Generally, infection rates and disease severity are highest among children younger than 5 years and particularly those younger than 2 years of age. During the...

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