Sty

• Poor hygiene that exposes your eyelids to bacteria. A sty is commonly caused by the bacterium staphylococcus, though other bacteria can also cause a sty. Touching your eyes with unwashed hands can transfer bacteria to your eyelids.
• Eyelid inflammation. Chronic inflammation along the edge of the eyelid caused by a condition called blepharitis can cause a sty to form. Following your doctor's instructions for treating your blepharitis can help prevent sties.

Risk factors

You risk exposing your eyelids to infection if you:

• Change your contact lenses without thoroughly washing your hands first
• Fail to disinfect your contact lenses before putting them in
• Leave eye makeup on overnight
• Use old or expired cosmetics

Diagnosis

Tests and procedures used to diagnose a sty include:

• Examining your eyelid. Your doctor may determine you have a sty by looking at your eyelid. Your doctor may use a light and a magnifying device to examine your eyelid.
• Testing for bacteria. If your doctor needs to lance your sty to relieve pressure, a small sample of pus may be collected for testing. This helps determine which bacterium is causing your infection and which antibiotics are most appropriate for you.

References

http://www.medicinenet.com/sty_stye/article.htm

http://www.webmd.com/first-aid/sty-treatment

http://www.mayoclinic.org/diseases-conditions/sty/basics/definition/con-20022698

https://www.hse.ie/eng/health/az/B/Blepharitis/