One thing about the flu shot is it does not cause the flu. Reactions, if any, are generally mild (headaches, nausea, swelling). But nothing is long term or severe. LouisEly's comment about getting a mild fever and sweats is an example of this. All that said, in very rare cases there can be an allergic reaction - which can cause all sorts of issues. But this is really rare. One big issue people have about the flu shot is understand what it does and doesn't do. It only protects you from 3-4 strains of influenza. That's it (and it's never 100% effective). There's lots more strains that can be out and about. It's not protecting you from - or causing - stomach flu, or bacterial infections, or the common cold, or a host of other things. And a flu shot doesn't become fully effective right away. It takes 2 weeks or so for the antibodies to build up in your body once you get the shot (which is important, because often times people go get the shot once they find out others they know have the flu - co-workers, kids, etc. Well, the protection you get from the shot won't kick in for a week or two, which doesn't help them if the flu is running rampant through your home or workplace.) In the end, we have to be sure we are not associating the flu shot with something else that occurs (meaning, I got a flu shot, and the the next day I got horrible diarrhea - so the flu shot must have cause that). I'm not saying it didn't cause it - but the odds are very, very low. If I have made any errors, people can update/correct.