A stye is usually caused by an infection with staphylococcus bacteria.

These bacteria often live on the skin without causing any harm.

External styes

An external stye (on the outside of your eyelid) may be caused by one of the following:

• an infection of an eyelash follicle (a small hole in your skin that an individual eyelash grows out of)
• an infection of the sebaceous (Zeis) gland – this gland is attached to the eyelash follicle and produces an oily substance called sebum, which lubricates the eyelash to prevent it drying out
• an infection of the apocrine (Moll) gland – this sweat gland empties into the eyelash follicle; the fluid joins the tear film that covers your eye and prevents the eye from drying out

Read more about staphylococcal infections.

Internal styes

An internal stye is caused by an infection of the meibomian gland. These glands are found on the eyelids and produce an oily liquid, which makes up part of the tear film that covers your eye.

Blepharitis

A stye can sometimes be a complication of another condition called blepharitis. Blepharitis causes the rims of your eyelids to become inflamed (red and swollen). This can result in symptoms such as:

• burning or sore eyes
• crusty eyelashes
• itchy eyelids

Blepharitis can be caused by a bacterial infection, or it can be a complication of a skin condition such as rosacea (a long-term condition that causes spots and redness of the face).

If you have chronic (long-term) blepharitis, you may be at increased risk of developing styes.