NoroVirus

You know that nasty illness that starts suddenly with vomiting and diarrhea, with maybe a slight fever, and has you running between the bathroom and bed, exhausted for several days? The one that I had, datapard had, and lots of others? The one that leaves you so rapidly dehydrated because you're afraid to drink even water because it bounces? The one that in really bad cases can have you passing out because of low blood pressure due to dehydration? It's not the flu.

It is norovirus:

The symptoms of norovirus illness usually include nausea, vomiting, diarrhea, and some stomach cramping. Sometimes people additionally have a low-grade fever, chills, headache, muscle aches, and a general sense of tiredness. The illness often begins suddenly, and the infected person may feel very sick. In most people the illness is self-limiting with symptoms lasting for about 1 or 2 days. In general, children experience more vomiting than adults. Most people with norovirus illness have both of these symptoms.
http://www.cdc.gov/ncidod/dvrd/revb/gastro/norovirus-qa.htm

If you're older, or have a weakened immune system, you may need to take special care, or go to the ER:

People may feel very sick and vomit many times a day, but most people get better within 1 or 2 days, and they have no long-term health effects related to their illness. However, sometimes people are unable to drink enough liquids to replace the liquids they lost because of vomiting and diarrhea. These persons can become dehydrated and may need special medical attention. This problem with dehydration is usually only seen among the very young, the elderly, and persons with weakened immune systems.

I suggest you read the info, and be ready to deal.

ETA: If you get it, force yourself to drink water, even if most of it comes back up. If you don't, you can start to feel "light headed" and even pass out because of dehydration. Besides, stomach acid alone is worse than diluted stomach acid. When I was in the puking/crapping stage the first day, I went through about three or so of those half-liter bottles of water, after I realized that I was starting to feel faint without anything to drink.