As mentioned, there have been some harsh flu pandemics in years past. Today, enough people contract the flu every year that if it were some other disease, it would be considered an epedemic.
Influenza is an interesting animal, it changes all the time, so if you got a flu vaccine last year, this year's flu bug will be a different animal...literally. There's some strains of the flu that hang around for many years...decades even. The CDC keeps track of the strains and issues monthly reports on all of them .
The "flu shot" is simply a collection of 5-12 flu vaccines. If you catch a strain of the flu that isn't in that year's vaccine, you're out of luck (but today, most people live..they're just under the weather for a bit). The "flu shot" minimizes the effects of the flu...some people still catch it, but they aren't in bed as much, etc. Two new "flu medicines" came out recently that aren't vaccines. They minimize the effects a tiny bit where someone could back to work an average of a day earlier.
Every year, a small group of doctors meets with the CDC to decide which strains will be included in next year's "flu shot." There's one doctor in New York, I don't recall his name, but he's one of the leading people in the field and has a big influence on the decision. Some years they successfully predict which flu strains will occur, other years the don't get so lucky and the "flu shot" has all the wrong strains in it. And in years like this year, the there's a screw up and the "flu shots" become contaminated.
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Which houses to raid? That's easy, they have a special sign in the front yard that reads "John Kerry for President."