The first is that while the flu vaccine is the best way to protect against the flu, it's far from perfect. In fact, the vaccine is only about 60 percent effective. So some people can get the flu even though they were vaccinated because the shot just didn't work for them.
The vaccine, for example, tends to work less well in the elderly. That's a big problem, since the elderly are among those at greatest risk for serious complications from the flu.
Another reason is that it takes about two weeks for the vaccine's protection to kick in. So if someone gets exposed to the flu in that time between when he gets the vaccine and when his immune system has responded sufficiently, he could still get sick.
Also, it's always possible that someone could get exposed to a strain of the flu virus that's not covered by the vaccine. This year's vaccine, though, looks like it should work pretty well — the flu virus strains in the vaccine appear to be a very good match for the most common flu strains that are circulating this year.
The last reason is that someone might get exposed to another kind of virus that causes symptoms that are very similar to the flu. There are lots of other viruses out there that can cause respiratory illnesses, such as adenoviruses, parainfluenza viruses and respiratory syncytial virus.