Mastoiitis is an infection of the mastoid process, the portion of the temporal bone of the skull that is behind the ear. It is usually caused by untreated acute otitis media (middle ear infection) and used to be a leading cause of child mortality. With the development of antibiotics, however, mastoiditis has become quite rare in developed countries, most likely due to antibiotic treatment of otitis media before it can spread. It is treated with medications and/or surgery. If untreated, the infection can spread to surrounding structures, including the brain, causing serious complications.