A stye is caused by a bacterial infection, most commonly involving the Staphylococcus bacteria, which infects an oil gland or hair follicle at the base of an eyelash or within the eyelid. This infection often occurs when the oil gland becomes blocked, leading to a painful red bump resembling a pimple. Styes can also be triggered by poor hygiene, rubbing the eyes, sleep deprivation, sharing contaminated items like pillowcases or cosmetics, and certain skin or medical conditions. The bacteria can be transferred from the nose to the eye by touching or rubbing, making styes somewhat contagious through shared contact with infected items.