When a person has a cold or flu, the Eustachian tube can become blocked by mucus or from swelling. If this happens, the Eustachian tube can't open to allow air to get to the middle ear. If the Eustachian tube remains plugged, fluid from the middle ear isn't able to drain and starts to build up in the middle ear, causing an ear infection. The medical term for ear infection is otitis media.