It's a pimple or abscess that forms on your upper or lower eyelid.
Sometimes the bacteria that normally live on the surface of your eyelid block an oil duct. Then it gets inflamed. Other times, germs and dead skin cells get trapped on the edge of your eyelid.
Most of the time a stye starts as a pimple next to an eyelash. It turns into a red, painful bump that can last several days before it bursts and then heals. Some styes are short-lived and heal on their own. Others may require a doctor’s care.
Styes are usually on the surface of your eyelid and easy to see. But they can form deep inside your eyelid. An internal stye (on the underside of your lid) also causes a red, painful bump. But its location prevents a whitehead from showing up on your eyelid. This type can also go away once the infection is gone. Some leave a small fluid-filled cyst that your doctor will have to cut open and drain.
What Causes Styes?
Usually it’s a combination of a clogged oil gland and a certain type of bacteria. Your body is coated with billions of friendly bacteria that live right along with you. Most of the time there’s no problem. But when conditions are right, the bacteria overproduce and create a pimple.
What’s a Chalazion?
If the clogged gland that produces the stye never gets better, scar tissue forms around it. The pain goes away but a bump remains. Doctors call that a chronic chalazion (pronounced cha-LAY-zee-yon).
Styes and chalazia (that’s the plural of chalazion) are usually harmless. They rarely affect your eyeball or eyesight. Rarely they can cause severe infections of the face called cellulitis. See your eye doctor for any significant pain or drastic swelling/redness of the whole eyelid. They can happen at any age and tend to come back from time to time especially in people who have ongoing eyelid irritation (blepharitis) or a skin condition called rosacea.