What exactly is "stomach flu"? Is it caused by a virus? What are the usual symptoms, and what are some of the preventive measures and cures for stomach flu?

M.Y.

"Stomach flu" is a common term, but in reality there is no medical condition with that name. Flu, which is short for influenza, refers to a virus that primarily causes respiratory illness. There are actually three influenza viruses (conveniently named A, B and C), but they cause pretty much the same syndrome, so it is generally not worth distinguishing them. As you probably know, influenza causes fever, severe muscle aches, cough, malaise (general feeling of illness) and weakness. Runny nose or other nasal symptoms are common. In its classic form, influenza is rather severe, causing most people to miss school or work. Colds are milder illnesses and are generally caused by other viruses.

When people say "stomach flu" or "intestinal flu," they are usually referring to an illness causing vomiting and/or diarrhea, a syndrome physicians call gastroenteritis. These illnesses can be caused by many different microbes, most of them viruses. In young children, the rotavirus is responsible for most cases of gastroenteritis. In older children and adults, common agents are the Norwalk virus and certain adenoviruses.

Occasionally, vomiting and diarrhea has a more serious cause, usually a bacterial infection. The bacteria that cause gastroenteritis include E. coli (especially a form found in partially cooked ground beef), salmonella (usually found in poorly cooked poultry products) and campylobacter (also found primarily in poultry).

How do you get gastroenteritis? Most of the viral cases and some bacterial cases are acquired by "fecal-oral transmission" -- that is, when a small amount of feces carrying the microbe is unknowingly consumed. This usually occurs via feces-contaminated food and other items put in the mouth. In the setting of a nursery, transmission from diaper to mouth may be more direct. Other cases, especially those caused by bacteria, come from so-called "food poisoning." This often occurs because livestock may carry organisms such as salmonella that can contaminate meat. Poor food-handling practices and insufficient cooking allow the bacteria to survive to infect an unsuspecting eater.

How can you prevent gastroenteritis? Handwashing helps. Wearing gloves while preparing food, especially in a restaurant setting, may also be a good idea. Properly cooking meat will minimize the risk of bacterial infection. This is particularly important for ground beef. Bacteria are usually found on the surface of the meat. When you cook a solid piece of meat such as a steak, the surface gets hot enough to kill the bacteria even if the interior is less well done. But when meat is ground up, surface bacteria are mixed throughout. Bacteria in the center of a ground-beef patty can survive unless the burger is cooked thoroughly. This is not to say that it's a good idea to eat very rare steaks -- but it is much worse to eat rare burgers.

Most cases of gastroenteritis are mild and go away by themselves. There is no need to treat any of the viruses and many of the bacteria. Getting rest and plenty of fluids is usually sufficient. However, seek medical attention if you have significant abdominal pain or blood in the stool. In addition, seek help if you cannot keep down any liquids or if you develop signs of dehydration -- including dizziness on standing, decreased urination and very dry mouth. High fever also requires attention, particularly if it is above 102.5 degrees F. (39.2 degrees C.) with diarrhea.