The basic underlying problem causing most forms of otitis is eustacian tube dysfunction. Most otitis occurs in patients whose eustacian tube, the tube between the nose and the middle ear (the area behind the eardrum), does not work properly. When air cannot adequately get through this tube to the middle ear,the negative pressure created can "suck" fluid out of the lining of the middle ear/mastoid, filling the middle ear and mastoid air cells with fluid. A mild hearing loss usually accompanies the fluid. The hearing loss disappears when the fluid is gone as long as there are no other causes for the hearing loss. Three kinds of otitis can result from eustacian tube dysfunction. They are serous and secretory otitis, where fluid fills the middle ear and mastoid, acute otitis, where pus fills the middle ear and mastoid but its presence is of short duration, and chronic otitis, where pus fills the middle ear and mastoid and it has been present for months or years. Chronic otitis is associated with infection of the bone itself and thickening and polyp formation of the mucosal lining of the middle ear and mastoid. The highest incidence of otitis media occurs in preschool children and decreases gradually after age 6. The highest incidence occurs poor children, children in day care, and Native Americans. Additional factors that cause or aggravate otitis include the presence of enlarged adenoid tissue, lack of proper muscle in the back of the throat (as in those with a cleft palate), allergy, immune deficiencies, sudden change in atmospheric pressure (like poor pressurization in an airplane dropping from a high altitude), scarring or tumors in the nasopharynx, and abnormal cell function of the mucosa of the ear and nose. Diabetes does not increase the incidence of otitis, but can make it much more difficult to treat.

Serous otitis, where fairly clear fluid fills the middle ear and mastoid, occurs with fairly sudden obstruction of the eustacian tube. A sudden descent of an airplane with poor pressurization or a bad cold are two of the most common causes of acute serous otitis media. Usually decongestants will clear the fluid or even blood that can be sucked from the mucosa into the middle ear with wither of these processes. If the fluid does not clear within a few weeks, it is considered chronic serous otitis. Older people with poorly functioning eustacian tubes commonly have recurrent serous otitis and may require intermittent tube placement over many years. Hearing loss is present depending on the amount of fluid in the ear. The hearing loss usually resolves when the fluid is cleared out of the ear, either medically or surgically.