There are a few different causes of a middle ear infection (otitis media): Eustachian Tube Blockage: If the eustachian tube (auditory tube) becomes blocked by swollen adenoids, any fluid and bacteria trapped inside can lead to an infection. Bacterial causes: Streptococcus pneumoniae and Haemophilus influenzae are two types of bacteria that often cause a middle ear infection. Viruses: The common cold and influenza (flu) are viruses that can lead to a middle ear infection.