Stomach flu is a general phrase describing vomiting and/or diarrhea. It is a catchy term that has stuck over the years. The more correct medical name is gastroenteritis. The phrase “stomach flu” is a misnomer because this illness is not caused by or in any way related to the infection influenza.

Most types of stomach flu are extremely contagious, so siblings and parents are likely to get sick as well. The typical incubation period is 48 to 72 hours. Infants tend to have more diarrhea and less vomiting; adults sometimes report only nausea, but it is so intense that they can lose their appetite for days.

The stomach flu usually begins with vomiting that lasts any¬where from 12 to 24 hours. This can vary widely, ranging from sporadic vomiting every couple of hours to intense repeated vomiting. Eventually, the contents of the stomach may be emptied, and your child will begin dry heaving, with nothing coming up.

Diarrhea typically follows vomiting by about a day or so, but the two can occur simultaneously, and in some cases the diarrhea even precedes the vomiting. Diarrhea means watery and frequent stools. The stools are not just soft; they are liquid. The diarrhea seen with most forms of stomach flu is voluminous, watery, foul-smelling, and associated with crampy abdominal pain. It will gen¬erally last anywhere from three to seven days, but it can persist for up to two weeks before the stools normalize completely.

Generally speaking, diarrhea is caused by intestinal-wall swelling. This interferes with normal absorption, leaving in the intestine water and nutrients that usually move out of the gut and into the bloodstream. This extra water gives the diarrhea its char¬acteristic liquid consistency. Significant diarrhea also changes the normal balance of bacteria in the gut. It is important to remember that bacteria are normal inhabitants of the intestine. Bacteria help break down food and produce byproducts (such as vitamin K) that are used by the rest of the body. When the number and types of bacteria change dramatically, the consistency of the stool will change, too.

Anything that inflames the intestine can cause diarrhea. Pos¬sible sources of inflammation include food allergy, diseases of the immune system, or infection. The most common virus to cause diarrhea among young children is rotavirus. Other viruses that can be to blame for stomach flu include astrovirus, adenovirus, and Norwalk virus. Bacteria such as salmonella, shigella, Cam¬pylobacter, and E. coli also can cause gastroenteritis. Worldwide, the most infamous bacterial cause of stomach flu — and one of the leading causes of death in developing countries — is cholera.

Ironically, the medicines that treat bacteria — antibiotics — can cause diarrhea, too. Antibiotics can change the balance of bacte¬ria that normally reside in the intestine or can cause an allergic reaction. Each may result in profuse diarrhea.

Regardless of the underlying cause, once diarrhea starts, it can be difficult to stop. This is because the intestine becomes very irritated. The more irritated it is, the more blunted the normally absorptive surfaces become. It takes time for these surfaces to return to their baseline. Especially in the case of an intestinal infection, it is often best to let diarrhea run its course rather than \ try to stop it with medicine. The sooner the infection has left the gut, the sooner the gut can begin to heal.