I hear this question a lot. Unfortunately, there is almost always some type of gastrointestinal virus ("stomach flu") circulating. These GI bugs cause really unpleasant symptoms like vomiting, diarrhea and fevers and they are highly contagious. If one person in the house gets sick, it is very difficult to keep everyone else healthy.
These stomach bugs (they really aren't related to the real flu at all) can be caused by a number of different viruses, the most common being norovirus. Outbreaks of norovirus have been known to sicken hundreds of people at a time, especially when people are in close proximity to one another for extended periods of time - such as on cruise ships and in schools.
Most of these viruses are relatively short lived. The vomiting shouldn't last more than about 24 hours and the diarrhea typically lasts up to a week after that. However, you need to be careful to watch for signs of dehydration and also monitor for other symptoms such as severe headache or constant abdominal pain (that isn't relieved when you vomit or have diarrhea) that could indicate something more serious than a virus is causing your symptoms.
If you do get one of these stomach bugs, there isn't a magic pill that you can take that will cure the virus, but there are a lot of things you can do to limit the symptoms - and chances are good that you are doing at least some of these things wrong because most people do.
If you get sick with symptoms of the stomach flu and you have concerns, contact your health care provider. Most people don't need any real treatment other than managing the symptoms at home, but some people do.