The reason for its development is that the sebaceous glands or hair follicles are richly distributed because of the blockage of the follicular ducts, which causes the sebaceous glands to secrete and accumulate in the hair follicles and then stimulate the formation of cyst-like swellings called sebaceous cysts; if accompanied by bacterial infection, local redness, swelling, heat, pain and even pus symptoms usually appear. If it is accompanied by bacterial infection, localized redness, swelling, heat, pain, and even pus may occur. The reason why sebaceous cysts smell bad after extrusion is because the contents are sebaceous secretions that accumulate for a long time, wrapping them and forming a closed environment, so they are accompanied by local bacterial growth and reproduction, and therefore smell bad.