Influenza viruses appear as three types. Type A infects a variety of birds and mammals, whereas B and C infect only humans. A and B cause epidemics; C is much less severe. Two proteins distinguish strain A subtypes: 15 variants of hemagglutinin (H), and nine of neuraminidase (N). A flu vaccine includes B and the most common A subtypes, H1N1 and H3N2; this year's is H1N1 (Caledonia), H3N2 (Panama), and strain B (Hong Kong). Flu viruses change in two basic ways. Antigenic shift is a sudden alteration of a hemagglutinin or neuraminidase; antigenic drift is the backdrop of continual mutation. Strain A shifts and drifts; B and C only drift.

"I had a birdy, his name was Enza. I opened the window, and in flew Enza."

- Children's rhyme popular during the influenza epidemic of 1918

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