A stye in the eye is primarily caused by a bacterial infection, most commonly by the bacterium Staphylococcus aureus. The infection occurs when bacteria infect an oil gland or hair follicle at the base of an eyelash or within the eyelid, leading to a small, painful lump. This can result from blocked glands or follicles, often due to factors such as touching or rubbing the eyes with unclean hands, using old or contaminated eye makeup, blepharitis (inflammation of the eyelids), or underlying conditions like rosacea, eczema, or diabetes. There are two main types of styes: external (forming on the outer edge of the eyelid) and internal (forming inside the eyelid). Styes are not generally contagious but can spread bacteria if the eye is touched without proper hand hygiene.