Although it's commonly called stomach flu, gastroenteritis isn't the same as influenza. Real flu (influenza) affects only your respiratory system — your nose, throat and lungs. Gastroenteritis, on the other hand, attacks your intestines, causing signs and symptoms, such as:

• Watery, usually nonbloody diarrhea — bloody diarrhea usually means you have a different, more severe infection
• Abdominal cramps and pain
• Nausea, vomiting or both
• Occasional muscle aches or headache
• Low-grade fever

Depending on the cause, viral gastroenteritis symptoms may appear within one to three days after you're infected and can range from mild to severe. Symptoms usually last just a day or two, but occasionally they may persist as long as 10 days.

Because the symptoms are similar, it's easy to confuse viral diarrhea with diarrhea caused by bacteria, such as Clostridium difficile, salmonella and E. coli, or parasites, such as giardia.

You're most likely to contract viral gastroenteritis when you eat or drink contaminated food or water, or if you share utensils, towels or food with someone who's infected.

A number of viruses can cause gastroenteritis, including:

• Noroviruses. Both children and adults are affected by noroviruses, the most common cause of foodborne illness worldwide. Norovirus infection can sweep through families and communities. It's especially likely to spread among people in confined spaces. In most cases, you pick up the virus from contaminated food or water, although person-to-person transmission also is possible.
• Rotavirus. Worldwide, this is the most common cause of viral gastroenteritis in children, who are usually infected when they put their fingers or other objects contaminated with the virus into their mouths. The infection is most severe in infants and young children. Adults infected with rotavirus may not have symptoms, but can still spread the illness — of particular concern in institutional settings because infected adults unknowingly can pass the virus to others. A vaccine against viral gastroenteritis is available in some countries, including the United States, and appears to be effective in preventing the infection.

Some shellfish, especially raw or undercooked oysters, also can make you sick. Although contaminated drinking water is a cause of viral diarrhea, in many cases the virus is passed through the fecal-oral route — that is, someone with a virus handles food you eat without washing his or her hands after using the toilet.

Gastroenteritis occurs all over the world, affecting people of every age, race and background.

People who may be more susceptible to gastroenteritis include:

• Young children. Children in child care centers or elementary schools may be especially vulnerable because it takes time for a child's immune system to mature.
• Older adults. Adult immune systems tend to become less efficient later in life. Older adults in nursing homes, in particular, are vulnerable because their immune systems weaken and they live in close contact with others who may pass along germs.
• Schoolchildren, churchgoers or dormitory residents. Anywhere that groups of people come together in close quarters can be an environment for an intestinal infection to get passed.
• Anyone with a weakened immune system. If your resistance to infection is low — for instance, if your immune system is compromised by HIV/AIDS, chemotherapy or another medical condition — you may be especially at risk.

Each gastrointestinal virus has a season when it's most active. If you live in the Northern Hemisphere, for instance, you're more likely to have rotavirus or norovirus infections between October and April.

The main complication of viral gastroenteritis is dehydration — a severe loss of water and essential salts and minerals. If you're healthy and drink enough to replace fluids you lose from vomiting and diarrhea, dehydration shouldn't be a problem.

Infants, older adults and people with suppressed immune systems may become severely dehydrated when they lose more fluids than they can replace. Hospitalization might be needed so that lost fluids can be replaced intravenously. Dehydration can be fatal, but rarely.