You can be contagious from a few days up to two weeks or more, depending on which virus is causing your stomach flu (gastroenteritis).

A number of viruses can cause gastroenteritis, including noroviruses and rotaviruses. The contagious period — the time during which a sick person can give the illness to others — differs slightly for each virus.

• Norovirus. With norovirus — the most common cause of viral gastroenteritis in adults — you're contagious when you begin to feel ill. Symptoms usually appear within one to two days of exposure.

  Although you typically feel better after a day or two, you're contagious for a few days after you recover. The virus can remain in your stool for up to two weeks or more after recovery.

  Children should stay home from school or child care for at least 48 hours after the last time they vomit or have diarrhea.

• Rotavirus. Symptoms of rotavirus — the leading cause of viral gastroenteritis in infants and young children — usually appear one to three days after exposure. But you're contagious even before you develop symptoms, and up to two weeks after you've recovered.

The viruses that cause gastroenteritis are spread through close contact with infected people, such as by sharing food or eating utensils, and by touching contaminated surfaces and objects. Eating contaminated food also can cause norovirus.

Washing your hands often with soap and water is the most effective way to stop the spread of these viruses to others. If you can't wash your hands, use an alcohol-based hand sanitizer, which can reduce germs.

To help keep others from getting sick, disinfect contaminated surfaces immediately after someone vomits or has diarrhea. Wear disposable gloves, and use a bleach-based household cleanser or 2 cups (0.5 liters) of bleach in a gallon (3.8 liters) of water. Norovirus can survive for months on surfaces not adequately disinfected with bleach solution.

Also wear disposable gloves to immediately wash clothes or linens that might be contaminated.

Two oral rotavirus vaccines are available for young infants — RotaTeq and Rotarix. Vaccines for norovirus are in clinical trials.