Mastoiditis is a bacterial infection of the mastoid bone. The mastoid bone, which sits behind the ear, consists of air spaces that help drain the middle ear. Specifically, it is an inflammation of the mucosal lining of the mastoid antrum and mastoid air cell system inside the mastoid process. Mastoiditis can develop when the mastoid cells become infected or inflamed, often as a result of a persistent middle ear infection. This can cause the porous bone to break down. Although the condition is most common in children, it can also occur in adults. The number of cases of mastoiditis has decreased significantly since the discovery of antibiotics in the early 1900s.