Welcome to Healthcare magic. 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, deafness occurs. If pressure build-up is too greate, sometimes the eardrum ruptures, and pus drains out. But more commonly, the pus and mucus remain in the middle ear due to the swollen and inflamed eustachian tube. Wishing your friend a speedy recovery.