Some infectious diseases, like smallpox and polio, can be prevented with a single vaccination. Influenza, however, is trickier to outsmart, since this virus can easily mutate. Even if your body has built up antibodies for one kind of flu, it doesn't mean that it has protection against all kinds of flu. The flu virus is like a moving target. So each year scientists try to predict what type of flu virus will be traveling to the U.S. Sometimes the scientists create the right vaccine and many people are protected; other times they're wrong and the flu wins.