<s>[INST] What causes an ear infection? [/INST] An ear infection usually is caused by bacteria and often begins after a child has a sore throat, cold, or other upper respiratory infection. If the upper respiratory infection is bacterial, these same bacteria may spread to the middle ear; if the upper respiratory infection is caused by a virus, such as a cold, bacteria may be drawn to the microbe-friendly environment and move into the middle ear as a secondary infection. Because of the infection, fluid builds up behind the eardrum. Source: NID CD NIH . Link: https://www.nidcd.nih.gov/health/ear-infections-children#:~:text=Many%20doctors%20will%20prescribe%20an,help%20with%20fever%20and%20pain.. Field: Otolaryngology. Disease: Ear Infection </s>