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. Demodex has been identified as a cause of blepharitis. If you have blepharitis, you are more likely to get styes.Other possible common eyelid bumps include:SymptomsIn addition to the red, swollen bump, other possible symptoms of a stye include:Tenderness of the eyelid,A gritty, scratchy sensation, as if there is a foreign body in your eye,,Sensitivity to light,Tearing of your eye. How is a stye diagnosed?A doctor can usually diagnose a stye by looking at it. They may also use a light or magnifying device to examine it more clearly or “flip” the eyelid to look underneath it before making a diagnosis.However, no special tests or exams are usually needed to diagnose a stye.