The eustachian tube connects the throat and middle ear. You have two tubes, one for each ear. This tube drains fluid from the middle ear into the throat. It also helps keep pressure in the middle ear equal.

During a cold, sinus or throat infection, or an allergy attack, the lining of the eustachian tube can swell and block the tube. Air can't move into or out of the middle ear, so a vacuum effect occurs, which pulls fluid into the middle ear. The fluid builds up in the ear because it can't drain down the throat. Germs can grow in the fluid and cause an infection. This condition is called acute otitis media (ear infection).