Infection of the middle ear (infection behind the ear drum). This infection causes infected fluid (pus) to collect in the middle ear. The middle ear contains the small bones of the ear. Otitis media is the most common cause of earaches. It is usually caused by bacteria, but it can start after a virus or allergy causes the ears and sinuses to be congested. Most infections heal on their own without antibiotics, as long as the infected fluid finds a path by which it can drain. In most cases, the middle ear drains through the eustachian tube, to the back of the nose. Occasionally, an ear infection drains through a hole that forms in the ear drum (perforated or ruptured ear drum).