What causes middle ear infection?

Bacterial and viral . Ear infections are more common in children due to their anatomy and predisposition for upper respiratory infections. When the eustachian tube fails to maintain a neutral pressure relative to the outside world, the middle ear can accumulate fluid, siphon bacteria from the upper respiratory tract into the middle ear and an infection results. Eustachian tube swelling is often initiated with a virus.
Sequential event. This is the result of several things happening over a period of time. First, natural mucous from the walls inside the middle ear become trapped by a eustachian or drainage tube that has become blocked. Then normal throat or mouth germs get into the mucous & begin to multiply in great numbers due to the availability of nutrients.As the numbers increase, pain & fever often begin.