A stye is caused by a blockage of one of the oil glands in the eyelids. This allows bacteria to grow inside the blocked gland. Styes are a lot like common acne pimples that occur elsewhere on the skin. You may have more than one stye at the same time.
Styes most often develop over a few days. They may drain and heal on their own. A stye can become a chalazion, which occurs when an inflamed oil gland becomes fully blocked. If a chalazion gets large enough, it can cause trouble with your vision. Styes can be made worse by the presence of Demodex, a mite commonly found on human skin.