Middle ear infections are caused by bacteria and viruses.

During a cold, sinus, or throat infection or an allergy attack, the eustachian tubes, which connect the middle ear to the throat, can become blocked. This stops fluid from draining from the middle ear. This fluid is a perfect breeding ground for bacteria or viruses to grow into an ear infection.

• Bacterial infections. Bacteria cause most ear infections.¹ The most common types are Streptococcus pneumoniae (also called pneumococcus), Haemophilus influenzae, and Moraxella catarrhalis.
• Viral infections. Viruses can also lead to ear infections. The respiratory syncytial virus (RSV) and influenza (flu) virus are the most frequent types found. These viruses account for the rise in ear infections from January to May each year.²

Causes of fluid buildup

When swelling from an upper respiratory infection or allergy blocks the eustachian tube, air can't reach the middle ear. This creates a vacuum and suction, which pulls fluid and germs from the nose and throat into the middle ear. The swollen tube prevents this fluid from draining. An ear infection begins when bacteria or viruses in the trapped fluid grow into an infection.

Inflammation and fluid buildup can occur without infection and cause a feeling of stuffiness in the ears. This is known as otitis media with effusion.