More commonly known as a middle ear infection, otitis media involves fluid buildup in the middle ear that becomes infected with a germ such as a virus or bacteriu. Otitis media is among the most common illnesses affecting children. Symptoms may appear in one or both ears.

Each year, over five million cases of otitis media occur in U.S. children, according to the American Academy of Pediatrics (AAP). More than 75 percent of children usually have an ear infection by their third birthday, and over half have several ear infections before they are 12 years old, according to the National Institute on Deafness and other Communication Disorders. Children typically stop having ear infections by the time they reach school age.

The middle ear is a chamber behind the eardrum (tympanic membrane). It is about the size of a pea. A narrow passageway called the eustachian tube connects the back of the middle ear to the back of the nose and throat. The tube equalizes the pressure inside and outside the ear and drains fluids from the middle ear. The inner ear structures include the cochlea, which transmits sounds to the brain, and the labyrinth, which regulates balance.

A cold, allergy or other condition can cause the eustachian tube to become inflamed, which can partially or fully block the passageway. Children are particularly susceptible to such occurrences because their eustachian tubes are typically narrower and shorter than those of adults. When this blockage occurs, fluid that builds up as a result of a cold or allergy (just as it does in the nose) has nowhere to drain. This can cause a temporary decrease in the child’s hearing.

In most cases, this fluid clears on its own within weeks and creates no lasting symptoms. However, in some cases the fluid lingers, becoming a breeding ground for the viruses and bacteria that are trapped due to the blockage of the eustachian tube. The result is an ear infection.  

Acute otitis media is the most common form of ear infection. It involves pus-filled fluid in the middle ear that causes pain, redness of the eardrum and fever. Chronic otitis media occurs when the fluid is present for more than six weeks. Otitis media with effusion involves fluid buildup in the middle ear that is temporary (usually lasting less than three months, and only rarely lasting a year or longer) and that often does not involve infection.

In some cases, fungi or bacteria may cause an infection of the external ear, known as otitis externa or swimmer ear. In very rare cases, the inner ear may become infected. Ear infections can also spread to deeper tissues and to the mastoid bone, which is located behind the ears. This condition is known as acute mastoiditis.