The big problem with flu vaccines is that the flu constantly mutates. Basically, it works like this: You catch the flu. You're immune system sends out white blood cells to fight the flu. The cells fight it, reverse engineer it's genetic code and embeds that code into your immune system, remembering it so it no longer affects you. But the flu has encoded a surprise in the virus. Once you're "over it", it uses you body's own immune system to dispose of the "dead" flu (sneezing, coughing, etc.). Those "dead" cells actually aren't
really dead, just a weakened form of the flu, one that has also learned from the fight with the immune system, upgraded to get past the defenses it ran into before. So, those disposed cells now go out and infect a new host. And the vicious cycle continues.

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