Pus behind the ear may be sebaceous cyst with infection, folliculitis, lymphadenitis, etc. 1. Sebaceous cysts: when the sebaceous glands of the hair follicles are blocked, sebum accumulates into cysts, and bacterial infections can appear behind the ears of patients with pus, accompanied by local redness and swelling. 2. Folliculitis: this disease may be caused by bacteria, fungi and other pathogens, but also may be caused by allergic reactions, mite parasites, low immune function, resulting in patients with pus behind the ears, accompanied by pain. 3. Inflammation of lymph nodes: this disease is mainly caused by bacterial infection of lymph node inflammation, of which hemolytic streptococcus and staphylococcus aureus are more common, which will lead to pustules behind the ears, accompanied by swollen lymph nodes and pain. It is recommended that the patient go to a regular hospital in time, after the doctor has made a clear diagnosis, according to the patient’s specific condition, to take targeted treatment.