Gastritis is an inflammation of the stomach lining that is usually caused by infection with Helicobacter pylori bacteria or long-term use of NSAIDs. It can occur suddenly or gradually. Symptoms include abdominal pain, nausea, and vomiting. Risk factors include H. pylori infection, alcoholism, smoking, older age, and inflammatory bowel disease. Diagnosis involves tests like an upper endoscopy, blood tests, and stool tests. Treatment focuses on eliminating the underlying cause, increasing fluid intake, avoiding irritants, and using medication if needed.