A stye is caused by a blockage of one of the sebaceous glands in the eyelids. This allows bacteria to grow inside the blocked gland. Sties look a lot like common acne pimples that appear on other parts of the skin. You may have more than one stye at the same time.
Sties are bacterial infections of a sebaceous gland or a hair follicle in the eyelid. Staph bacteria are the most common cause of styes. Sties are caused by skin bacteria (usually staphylococcal bacteria) that penetrate and irritate the sebaceous glands in the eyelids. These bacteria, which normally exist harmlessly on the skin of the eye, can sometimes be trapped along with dead skin cells on the edge of the eyelid.
The result is a swollen, red, painful bump that may develop over the course of a few days. It is a pimple or abscess that forms on the upper or lower eyelid.