A painful, tender stye (also called a hordeolum) is usually caused by a bacterial infection. A stye will develop at the base of an eyelash if the eyelash follicle (root) is infected. This is usually called an external hordeolum. A stye will also develop if there is an infection in one of the tiny lubricating oil glands (called meibomian glands) inside or under the eyelid. When this occurs, it is usually called an internal hordeolum. A stye can also be caused by widespread inflammation of the eyelid from blepharitis, a condition that affects the eyelid margins.
A chalazion develops when an oil gland in the eyelid becomes blocked. If an internal hordeolum doesn't drain and heal, it can turn into a chalazion. Unlike a stye, a chalazion is usually painless.
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