The most common cause of gastritis is infection by Helicobacter pylori (H. pylori). These small, curved bacteria may cause acute or chronic gastritis. They typically cause erosive gastritis, but they may be linked to other types as well. Unlike most bacteria, H. pylori are able to survive and even thrive in the highly acidic stomach environment. They grow in the mucous layer that is secreted by the stomach lining (gastric mucosa), which protects them from the digestive juices. The bacteria also produce large amounts of urease, an enzyme that breaks down to form ammonia. Ammonia neutralizes the acidity around the bacteria, further protecting them.

Gastritis that results from an infection of H. pylori may affect the entire stomach or just the lower part, or antrum. Other diseases, including peptic ulcers and stomach cancer, are also linked to chronic H. pylori infections. About two-thirds of the world’s population carries the H. pylori bacteria, according to the U.S. Centers for Disease Control and Prevention (CDC). However, not everyone with the bacteria develops gastritis or the other related conditions. In the United States, the bacteria occur more commonly among adults over age 60. Infection with other forms of bacteria (e.g., Escherichia coli, Salmonella) can also cause gastritis.

Some other potential causes of different types of gastritis include:

• Medications. Many medications may irritate the gastric mucosa and cause erosive gastritis. Nonsteroidal anti-inflammatory drugs (NSAIDs) are the most likely to cause this irritation, especially with chronic use. These pain relievers tend to cause gastritis in the antrum. The combination of NSAIDs and H. pylori can cause extensive damage to the stomach lining.
  
  
• Injury and stress. Illness, injury or extreme stress may cause erosive gastritis and acute stress gastritis.
  
  
• Disorders of the immune system. Problems with the immune response may cause a patient’s antibodies to attack the lining of the stomach. This generally results in atrophic gastritis.
  
  
• Crohn's disease. This is a type of inflammatory bowel disease. It usually affects the intestines, but in rare cases, it may affect the stomach, causing erosive gastritis.
  
  
• Viruses, fungi and parasites. Viruses, parasites or fungi do not usually infect the stomachs of healthy individuals, but may pose a problem to patients with weakened immune systems, such as those with acquired immune deficiency syndrome (AIDS) or patients taking immunosuppressant medications.
  
  
• Radiation. When radiation is delivered to the upper abdomen or lower chest, it may irritate the lining of the stomach and cause gastritis. Although this does not happen to all patients, physicians may recommend medications to protect the stomach lining before x-rays or radiation treatments of the upper gastrointestinal tract are performed in patients who may be at an increased risk for gastritis.
  
  
• Improper levels of digestive juices or reflux of bile. Too much hydrochloric acid in the stomach may result in erosive gastritis. This may also occur if bile from the small intestine flows back into the stomach.
  
  
• Hypersensitivities or allergies. Patients may be hypersensitive or allergic to foods or other stomach contents (e.g., parasites, infections). Such allergies or sensitivities often cause gastritis.
  
  
• Gastrectomy. Gastritis that occurs following the removal of part of the stomach (partial gastrectomy) is called postgastrectomy gastritis. Inflammation typically occurs where the tissues have been sewn back together and may be due to impaired blood flow or bile back-flow. Inflammation may also occur following a near-total gastrectomy, affecting the small amounts of gastric mucosa that are left.

Certain risk factors increase a person’s likelihood of developing gastritis. Because many gastritis cases are caused by H. pylori bacteria, people who carry the bacteria have an increased risk of developing gastritis. Other risk factors include:

• Substance abuse and certain diets. Excessive alcohol consumption, cigarette smoking or illicit drug use can weaken the stomach lining’s defenses against digestive acids. Diets rich in foods that are spicy, acidic, fried or fatty may also increase the risk of gastritis.
  
  
• Stress. Even minor stresses can increase the production of stomach acids and slow digestion, increasing the risk of gastritis.
  
  
• Race and ethnicity. Gastritis caused by the bacteria H. pylori is most common in the United States in individuals who are black, Native American or Hispanic. Gastritis due to problems with the immune system occurs most frequently in individuals who are black or of Northern European descent.