A stye is caused by bacteria from the skin that get into the oil glands in the eyelids that provide lubrication to the tear film. Styes are similar to common acne pimples that occur elsewhere on the skin. You may have more than one stye at the same time.
Styes usually develop over a few days and may drain and heal on their own. A stye can become a chalazion -- this is when an inflamed oil gland becomes fully blocked. If a chalazion gets large enough, it can cause trouble with your vision.
This post was edited on 10/11 at 9:50 pm