Mastoiditis is an infection of the mastoid bone, located behind the ear. It often occurs as a complication of an untreated middle ear infection or otitis media. Symptoms may include pain, swelling, and redness behind the ear, fever, and drainage from the ear canal. If left untreated, mastoiditis can lead to serious complications, such as hearing loss or the spread of infection to nearby structures. Treatment typically involves antibiotics, and in some cases, surgery may be necessary to drain the infected area and prevent further complications. Early diagnosis and treatment are crucial for a good outcome.