A stye is when the oil gland of the eyelid becomes inflamed. It may develop into an infection with a small pocket of pus (an abscess). This can cause pain, redness, and swelling. In early stages, a stye is treated with antibiotic ointment, eye drops, or a small towel soaked in warm water (a warm compress). More severe cases may need to be opened and drained by an eye specialist.