Wrong. As we explained in the previous section, a shot of flu vaccine can give you immunity for many months, and even as long as a year. But that's not forever. According to the Centers for Disease Control and Prevention, there are numerous studies, conducted over different flu seasons and with different types of vaccines and virus strains, which show that the body's immunity does indeed decline over time. And that holds true whether you're inoculated against the virus, or you catch it from somebody who sneezes on you on your morning commuter train [source: CDC].
To make matters worse, there isn't just one type of flu, and flu viruses don't just stay the same, year after year. Instead, these tiny microbes mutate and evolve, just as humans did. But it took complicated organisms like us millions of years to evolve, while viruses, which are much simpler, sometimes do it from one flu season to the next.
They have to in order to survive. Unlike plants, animals and humans, the only way a virus can reproduce itself is by invading a host cell and injecting its genetic material into it. That material then gives the cell instructions to make more copies of the virus, which then spreads to other cells. The host organisms don't go along with this willingly -- their immune systems learn to recognize the distinctive proteins on a particular virus coating and attack it. Thus, in order to overcome that obstacle, viruses continually have to reinvent themselves and become essentially new attackers that your body won't recognize [sources: CBC News, Sepsis Alliance]. That means you have to get annual inoculations to protect against these new threats.