Viral infection is one of the most common causes of gastroenteritis. The condition 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, although they can also be spread through contaminated food and water. A number of different viruses can cause gastroenteritis, including:

• Rotavirus. Leading cause of gastroenteritis in children ages 3 months to 15 months old. It can also affect adults who come into close contact with children, but produces milder symptoms in adults. Infections usually occur during the winter.
  
  
• Calicivirus. Causes infection in people of all ages. 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.
  
  
• 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.

Most bacteria that cause gastroenteritis are spread through contaminated food. 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. The bacteria live in the intestines of healthy birds.
  
  
• Salmonella. Found in the intestines of birds, reptiles and mammals.
  
  
• 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.
  
  
• 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 by consuming contaminated water and food. They can also be spread through contact with contaminated surfaces, toys and other objects, or by swimming in water infected by these organisms.

• Giardia lamblia. Parasite that invades the lining of the intestine and causes symptoms, such as nausea, vomiting and diarrhea. 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.

Some types of foods, such as shellfish and certain species of mushrooms, contain chemicals or toxins that result in illness when eaten. Most cases result in mild symptoms, but some are more serious and require immediate medical treatment.

In addition, there are many medications that can cause gastroenteritis (drug-related gastroenteritis) and small intestinal inflammation. 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. Other medications, such as antacids, laxatives, aspirin and nonsteroidal anti-inflammatory drugs (NSAIDs) can irritate the stomach.

People at greater risk of developing gastroenteritis include:

• Elderly people. As people age, their immune systems do not respond as quickly and effectively to infectious organisms.
  
  
• Infants and young children. The immune systems of very young people are not fully developed.
  
  
• People with weakened immune systems. People with certain medical conditions, such as human immunodeficiency virus (HIV) or diabetes, have a greater risk.
  
  
• Pregnant women. Pregnant women are at greater of getting gastroenteritis.