Viral stomach flu or also know as stomach virus is for sure the most frequent trigger of stomach flu. More than 30% of stomach flu comes from viral infection. There are many potentially dangerous viruses and the most known are norovirus and rotavirus. Soon after them comes adenoviru, astroviruss and sapovirus. Each of them needs to be explained, so I'll take some time to write separate posts about them in future. Viruses are a bit faster than other causes, because of that period incubation is from 1 to 3 days. If you suffer from stomach virus, diarrhea, vomiting and headache are sure symptoms you'll experience during your sickness. Rest a lot, take some fluids, not to much, do not eat solid food and you should recover in several days.
Bacterial stomach flu is caused by bacteria. The most known bacteria that can cause this illness are salmonella, shigella, listeria, aeromonas, bacillus, plesiomonas, staphylococcus, vibrio and yersinia. The period of incubation for bacteria depend which bacteria is infected you and can take from 2 to 5 days before symptoms occur The symptoms are almost the same like with viral infection, just you'll may have them more frequent. It's the same treatment, rest a lot, some fluids intake, no food for first several days, avoiding coffee and alcohol, and cigarettes.
Parasitical stomach flu caused by parasites which leave in foods and water, where they multiply easy. The most known are entamoeba, giardia and cryptosporidium. Spreading is going very easy, water on public places such are pools can easily get contaminated. The symptoms are same just like with viral and bacterial infection, with one difference about period of incubation, which can take up to 2 weeks before symptoms apply.
Tip: watch out not to dehydrate while you have stomach flu, if you think you are, sick for medical help!