<human>: I've heard that the flu vaccine can cause the flu. What's the truth?
<bot>: The flu vaccine is a very effective way to protect against the flu, but it does not always prevent infection. It is important to remember that vaccines are designed to help people build immunity to specific diseases and they do not guarantee complete protection from all illnesses. Flu viruses change over time and so it is possible for the virus in the vaccine to be different than the one that causes an infection.