There are several common reasons for shoulder acne: first, warts. The so-called warts are caused by a viral infection and are contagious. It is possible to be infected when you take a shower inside the bathhouse, thus causing beans to grow on your shoulder. Second, sebaceous cysts. The reason is that the sebaceous glands secrete sweat, which can be discharged outside the skin through small ducts. If the ducts are blocked by sweat or surrounding tissues, the sweat secreted cannot be discharged outward, and the accumulation over time can cause sebaceous cysts, and sometimes multiple sebaceous cysts can co-occur.