A stye is a sore and red bump located near the edge of the eyelid.  It is caused by an infection at the base of the eyelash.  The usual culprit is a bacteria called Staphylococcal.  These bacteria normally live harmlessly on the skin, but can cause infection if the skin is damaged in any way.  If you touch mucus from your nose then rub your eye you can transfer this bacteria to you eyelid.

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Styes may be red and sore and can develop on either the front or backside of the eyelid.  Most Styes clear up in a few days, but it is recommended you see an eye doctor if you experience pain, it does not clear up within a few days, or has transferred to the other eye.