Blockage of the eustachian tube during a cold, allergy, or upper respiratory infection and the presence of bacteria or viruses lead to the accumulation of fluid (a build-up of pus and mucus) behind the eardrum. This is the infection called acute otitis media. The build up of pressurized pus in the middle ear causes earache, swelling, and redness. Since the eardrum cannot vibrate properly, you or your child may have hearing problems. Sometimes the eardrum ruptures, and pus drains out of the ear. But more commonly, the pus and mucus remain in the middle ear due to the swollen and inflamed eustachian tube. This is called middle ear effusion or serous otitis media. Often after the acute infection has passed, the effusion remains and becomes chronic, lasting for weeks, months, or even years. This condition makes one subject to frequent recurrences of the acute infection and may cause difficulty in hearing.