Gastritis is defined as inflammation of the lining of the stomach. This inflammation occurs when H. pylori or one of several other organisms infect the stomach. This is often associated with low levels of stomach acid. When stomach acid levels are very low, or chronically neutralized by antacids, H. pylori is less likely to be the cause and the overgrowth of others becomes more likely. Other causes, some of which act through low stomach acid, include medications, alcohol, ingestion of corrosive substances and extreme physiological stress. Acute gastritis is often associated with a sudden and severe illness or trauma. The incidence is about 8 out of every 1,000 people.

Sometimes the inflammation of the stomach lining can become so serious that an ulcer develops. Gastritis can be diagnosed by biopsy and specimen examination under a microscope, where the presence of increased white blood cells is seen.