Mastoiditis is an infection of the mastoid bone, located behind the ear. It often occurs as a complication of an untreated or severe middle ear infection, known as otitis media. Symptoms may include ear pain, swelling behind the ear, fever, and drainage from the ear. If left untreated, mastoiditis can lead to serious complications, such as hearing loss or the spread of infection to nearby areas, including the brain. Treatment typically involves antibiotics and, in some cases, surgery to drain the infected area and prevent further complications.