A variety of factors have the potential to cause gastroenteritis. Chief among these are viral infections, bacterial infections and parasitic infections. In some cases – particularly with viruses – these infections are spread through person-to-person contact. However, in many other cases, they are spread through contaminated foods (known as foodborne illnesses) or water. About 76 million people in the United States become ill every year from consuming foods contaminated by pathogens such as viruses, bacteria and parasites, according to the U.S. Centers for Disease Control and Prevention (CDC). Several thousand of these patients die as a result.

Viral infection is often called “stomach flu,” but it is not actually caused by an influenza virus (which is an upper respiratory disease). Viruses are typically spread through person-to-person contact, such as through saliva, hand-to-skin contact or sneezing. Children often contract a virus when they put their fingers in their mouths after touching something contaminated with the virus. Viruses can also be spread through contaminated food, surfaces and water. A number of different viruses can cause gastroenteritis, including:

• Rotavirus. A leading cause of gastroenteritis in children ages 3 months to 15 months, and the leading cause of diarrhea in children under age 5. Each year, 55,000 children in the United States are hospitalized due to rotavirus infection, according to the CDC. It can also affect adults who come into close contact with children, but produces milder symptoms in adults. Infections usually occur during the winter months. Even a small amount of virus can trigger serious illness. Children infected with a rotavirus are usually contagious for about 10 days afterward.
  
  
• Calicivirus. The most common source of gastroenteritis in the United States, these viruses are most likely to affect older children and adults. Caliciviruses are divided into noroviruses and sapoviruses. Epidemics (when a large group of people are infected at the same time or from the same source) are usually caused by caliciviruses that infect groups of people who live in confined areas. It most often is passed through contaminated water, feces and food, but sometimes is transmitted through person-to-person contact. Noroviruses can be contagious for varying periods of time, from three days to three weeks.
  
  
• Adenovirus. Causes gastroenteritis in children younger than age 2. Symptoms usually appear one week after exposure. Infections occur year-round.
  
  
• Astrovirus. Causes infection primarily in infants, young children and the elderly. Infections usually occur during the winter months.

Bacterial infections also cause gastroenteritis. Most bacteria that cause gastroenteritis are spread through contaminated food, a process commonly known as food poisoning. Bacteria are the chief source of contamination of food. Children can contract bacterial infection by contact with feces, eating mayonnaise, dairy products, meat or other foods that have been left unrefrigerated. Improper food handling and undercooked food are common causes of bacterial infection.

The most common types of bacteria that cause gastroenteritis include:

• Campylobacter. The most commonly identified bacterial cause of diarrheal illness in the world. Usually causes bloody diarrhea by the second or third day of the infection. The bacteria live in the intestines of healthy birds.
  
  
• Salmonella. Found in the intestines of birds, reptiles and mammals. Foods often contaminated with salmonella include eggs, lettuce, tomatoes, sprouts and melons.
  
  
• Escherichia coli (E. coli). Bacterial pathogen found in cattle and similar animals. Illness typically occurs after consumption of food or water that has been contaminated with microscopic amounts of cow feces. Children between the ages of 1 and 10 are particularly vulnerable to hemolytic uremic syndrome, an infection with a strain called E. coli O157:H7 that can cause kidney failure and death. Undercooked meat, unpasteurized apple cider or apple juice, and raw sprouts are the chief source of this bacteria.
  
  
• Staphylococcus aureus. Unlike other types of foodborne illness, this is not caused by contamination. This bacterium can grow in some foods and produce a toxin that results in intense vomiting.

Parasites and protozoans can cause gastroenteritis when a person consumes contaminated water and food, or comes in contact with contaminated feces. They can also be spread through a child’s contact with contaminated surfaces, toys and other objects, or by swimming in water infected by these organisms. Types associated with gastroenteritis include:

• Giardia lamblia. Parasite that invades the lining of the intestine and causes symptoms, such as nausea, vomiting and diarrhea. A giardia infection may also cause temporary lactose intolerance in some people. It is more common in cold climates, but can occur in any climate.
  
  
• Cryptosporidia. Causes watery diarrhea that is sometimes accompanied by abdominal cramps, nausea and vomiting. The infection is usually mild in healthy people, but can be severe or fatal in people with weakened immune systems.

Certain drugs can cause gastroenteritis, although this form of infection is not contagious to others. For example, antibiotics can alter the normal amount and type of bacteria found in the large intestine and allow the bacterium clostridium difficile to grow and cause diarrhea. Aspirin, nonsteroidal anti-inflammatory drugs (NSAIDs), steroids, laxatives and caffeine, among others, can irritate the stomach and cause gastroenteritis.

Other sources of gastroenteritis include the sugar lactose in milk and milk products (which causes lactose intolerance in some people), exposure to heavy metals (e.g., arsenic, lead and mercury) in drinking water, and chemicals or toxins in foods such as shellfish and certain types of mushrooms.

Foods that are new to a young child sometimes trigger bouts of gastroenteritis, and breastfeeding mothers sometimes eat foods that cause gastroenteritis when passed through breast milk to their child.

Infants and young children are at high risk for gastroenteritis, primarily because the immune system is not fully developed until approximately age 6. Children in developing nations are particularly at risk for acquiring the viruses that most often cause gastroenteritis. Children who attend child care facilities or schools, or who are often in enclosed areas with other children are also at increased risk. Others who may face a higher risk of developing gastroenteritis include:

• Elderly people
• People with weakened immune systems
• Pregnant women