An ear infection usually is caused by bacteria and often begins after a child has a sore throat, cold, or other upper respiratory infection. If the upper respiratory infection is bacterial, these same bacteria may spread to the middle ear; if the upper respiratory infection is caused by a virus, such as a cold, bacteria may be drawn to the microbe-friendly environment and move into the middle ear as a secondary infection. Because of the infection, fluid builds up behind the eardrum.
The ear has three major parts: the outer ear, the middle ear, and the inner ear. The outer ear, also called the pinna, includes everything we see on the outside—the curved flap of the ear leading down to the earlobe—but it also includes the ear canal, which begins at the opening to the ear and extends to the eardrum. The eardrum is a membrane that separates the outer ear from the middle ear.
The middle ear—which is where ear infections occur—is located between the eardrum and the inner ear. Within the middle ear are three tiny bones called the malleus, incus, and stapes that transmit sound vibrations from the eardrum to the inner ear. The bones of the middle ear are surrounded by air.
The inner ear contains the labyrinth, which help us keep our balance. The cochlea, a part of the labyrinth, is a snail-shaped organ that converts sound vibrations from the middle ear into electrical signals. The auditory nerve carries these signals from the cochlea to the brain.
Other nearby parts of the ear also can be involved in ear infections. The eustachian tube is a small passageway that connects the upper part of the throat to the middle ear. Its job is to supply fresh air to the middle ear, drain fluid, and keep air pressure at a steady level between the nose and the ear.
Adenoids are small pads of tissue located behind the back of the nose, above the throat, and near the eustachian tubes. Adenoids are mostly made up of immune system cells. They fight off infection by trapping bacteria that enter through the mouth.