Gastroenteritis, or inflammation of the gastrointestinal tract, is an acute condition that can cause abdominal pain, nausea, vomiting, and diarrhea. Inflammation may occur in the lining of the stomach, small intestine, and/or large intestine, and in some cases, people with the condition may become dehydrated or have other complications.

In the United States, about 1 out of every 6 people get gastroenteritis from eating contaminated food every year. Viral gastroenteritis is the most common cause of diarrhea worldwide.

Dehydration is the most common complication of gastroenteritis. Babies and young children, in particular, are more susceptible to complications because they can become dehydrated more easily than adults. As a result, many children require medical care, which often involves rehydration therapy. Older adults and people who are immunosuppressed are also at risk of dehydration from gastroenteritis.

Fortunately, the condition typically improves over several days without the need for medical intervention; most people recover on their own.

Only a small percentage of people with gastroenteritis need to be treated in the hospital, and it’s usually for dehydration or other complications of the condition—not for the condition itself.

Gastroenteritis is the medical term for inflammation of the stomach and intestines; viral gastroenteritis is commonly known as the “stomach flu” because people with the condition notice a sudden, drastic change in the way they feel, similar to that of the flu. They may quickly lose their appetite, become nauseous, experience diarrhea, or begin vomiting. Although foodborne illness is a major cause, there are other ways people can develop the condition.

It’s important to note that some cases are infectious, while others are not contagious based on the cause of gastroenteritis.

When people ingest viruses, bacteria, parasites, medication, or chemicals that lead to gastroenteritis, the presence of these pathogens in the digestive system causes inflammation in the lining of the stomach, large intestine, or small intestine. This inflammation causes gastrointestinal symptoms, including vomiting and/or diarrhea, which can lead to dehydration and other serious complications.

The condition is most commonly caused by viruses, but it may also be caused by bacteria, parasites, medication, and exposure to chemicals.

Viruses. The viruses that cause gastroenteritis spread via what’s known as “fecal-oral” transmission. The feces and vomit of a person who has gastroenteritis can be contaminated with the virus that causes the condition. If someone with viral gastroenteritis doesn’t sufficiently wash their hands after a bowel movement or vomiting, they may contaminate surfaces, food, water, or other beverages with the virus by touching them. The virus can spread to others when an uninfected person touches the contaminated surface, then touches their mouth, or when an uninfected person eats or drinks contaminated food or beverages, thereby introducing the virus into their body. People can also be exposed to and infected by gastroenteritis-causing viruses by swimming in contaminated water.

Viruses that cause gastroenteritis include:

• Norovirus
• Rotavirus
• Astrovirus
• Adenovirus

Bacteria. People usually come in contact with bacteria that cause gastroenteritis by consuming raw or undercooked food or unpasteurized foods or beverages. It’s also possible to pick up these bacteria by touching certain pets, including reptiles and birds, or by swimming in and/or swallowing contaminated water.

Bacteria that cause gastroenteritis include:

• Campylobacter
• Clostridioides difficile (C. diff)
• Escherichia coli (E. coli)
• Salmonella
• Shigella
• Yersinia

Parasites. Although this form of gastroenteritis is uncommon in the U.S., it can occur when eating or drinking contaminated food or water. In daycare settings, parasites may spread when caregivers don’t wash their hands properly after changing diapers. Gastroenteritis-causing parasites can also be transmitted through oro-anal sexual contact. Parasitic infections can be picked up while traveling as well.

Parasites that cause gastroenteritis include:

• Giardia intestinalis
• Cryptosporidium parvum
• Entamoeba histolytica

People who have gastroenteritis experience some or all of the following symptoms:

• Loss of appetite
• Nausea
• Vomiting
• Abdominal pain or cramping
• Bloating
• Diarrhea
• Blood or mucus in the stool
• Fever
• Fatigue
• Muscle aches
• Sweating
• Chills
• Weight loss

People may be at risk of gastroenteritis if they:

• Do not wash their hands properly after using the bathroom or changing a diaper
• Prepare food without washing their hands first
• Consume food that was improperly handled at a supermarket or restaurant
• Consume food that was prepared in a restaurant by someone who didn’t wash their hands
• Consume food that sat unrefrigerated for too long, allowing bacteria to proliferate
• Consume undercooked or unpasteurized foods
• Drink untreated well water or accidentally drink swimming water

You may be diagnosed with gastroenteritis after sharing your medical history with a doctor, getting a physical exam, and, in some cases, undergoing diagnostic testing.

When sharing your medical history, let the doctor know about your symptoms and how long they have been affecting you. Have you eaten anything unusual or traveled lately? Have you started taking a new medication? Are you able to keep down liquids? Do you feel exhausted? If other people in your home have had gastroenteritis recently, share that information, too.

During a physical exam, doctors look for signs of gastroenteritis, including fever, fatigue, bloating, and abdominal pain and cramping. Doctors may gently press on the abdomen to see if certain areas are more tender than others. If you have intense pain in one spot, doctors may suspect other conditions, not gastroenteritis. They will also check for signs of dehydration.

In some cases, you may undergo the following diagnostic testing:

• A stool test, for which you need to provide a stool sample for lab testing
• A sigmoidoscopy, during which the lower part of your large intestine will be examined; the doctor will insert a thin, flexible tube with a camera on its end into your anus
• Blood tests to rule out other problems or to determine if you have an electrolyte imbalance

Because vomiting and diarrhea can lead to dehydration, rehydration (plus bed rest) is the most effective treatment for people with gastroenteritis. Small sips of liquid every few minutes may be effective for people who are vomiting. An oral rehydration solution, such as Gatorade, that contains electrolytes is an ideal choice. People who become severely dehydrated may need to receive fluids intravenously (by IV) at the hospital.

In addition, the following medications may also be recommended:

• Anti-nausea medications, including ondansetron, prochlorperazine, or promethazine; they may be given in suppository form if you are unable to hold down liquids
• Anti-diarrheal drugs, like loperamide, if diarrhea lasts longer than a day or two; this treatment isn’t appropriate for patients aged 65 or older.
• Antibiotics may be prescribed for bacterial gastroenteritis
• Anti-parasitic drugs, like metronidazole or nitazoxanide, may be prescribed for parasitic gastroenteritis

For most people, gastroenteritis improves within a week. Typically, symptoms gradually fade with rehydration. Most people won’t require medication or hospitalization and will make full recoveries at home in a short time frame.

Young children, older adults, and immunosuppressed people may be at risk of complications or even death from gastroenteritis in rare cases, underscoring the importance of hydration. These patients may benefit from seeing a physician for care. Some may need to be admitted to the hospital for IV fluids. Under the supervision of a doctor, these patients are also expected to fully recover.

This article was medically reviewed by Yale Medicine infectious diseases specialist Matthew Grant, MD.