Styes are caused by Staphylococcus aureus, a common bacterium that lives on human skin and inside the nose. When this germ gets into one of the tiny glands along your eyelid, it triggers an infection that produces the painful, red bump you recognize as a stye. S. aureus is responsible for the vast majority of cases.
How S. Aureus Infects the Eyelid
Your eyelids contain dozens of small glands that produce oil to keep your eyes lubricated. An external stye forms when bacteria infect a hair follicle at the base of an eyelash or one of the small oil or sweat glands near it. An internal stye develops when bacteria reach the larger oil-producing glands deeper inside the eyelid. In both cases, the gland becomes blocked and inflamed, filling with pus as your immune system fights the infection.
S. aureus is especially good at colonizing skin. Roughly 30% of people carry it in their nose at any given time, and it transfers easily to the eyes when you rub your face or handle contact lenses. Once inside a gland opening, the bacteria multiply quickly in the warm, oily environment, producing the hallmark tender swelling within a day or two.
External vs. Internal Styes
External styes sit right along the lash line. They tend to come to a head like a small pimple and often drain on their own within a few days. Internal styes form deeper in the eyelid and can be more painful because the infection is trapped under thicker tissue. Internal styes are also more likely to need treatment beyond simple home care, sometimes requiring oral antibiotics or drainage by a doctor.
Both types are caused by the same bacterium. The difference is simply which gland gets infected.
Styes vs. Chalazia
A bump on the eyelid isn’t always a stye. A chalazion looks similar but develops when an oil gland clogs without a bacterial infection. It tends to be less painful, grows more slowly, and doesn’t produce the acute redness and tenderness of a stye. In the first couple of days, even eye doctors can have trouble telling the two apart. The key distinction: a stye is a bacterial infection, while a chalazion is a blocked gland. A stye that doesn’t fully resolve can sometimes turn into a chalazion over time.
Who Gets Styes
Styes are one of the most common eyelid problems doctors see, though exact prevalence numbers aren’t tracked. Men and women are affected equally, and there’s no difference across racial or ethnic groups. Adults get styes more often than children, likely because adult skin produces thicker, more viscous oil that clogs gland openings more easily. Conditions like rosacea and chronic eyelid inflammation also become more common with age, raising the risk further.
How Bacteria Spread to Your Eyes
S. aureus doesn’t need to come from someone else. It’s already on your skin. The most common route to a stye is simply touching your eyes with unwashed hands. But contaminated items play a real role too, especially makeup. Every time a mascara wand or eyeliner pencil touches your lid, bacteria can transfer to the product. The next time you use it, those bacteria go right back onto your eye.
Old makeup is a particular problem because bacteria multiply inside the container over time. Mascara and liquid eyeliner should be replaced every four months. Solid eye pencils last up to a year. Sharing eye makeup with others significantly increases the chance of introducing new bacteria to your lids.
Styes themselves aren’t contagious in the way a cold is. You won’t catch one by sitting next to someone who has one. But reusing your own contaminated products or towels can reintroduce bacteria and cause recurring styes.
How Styes Are Treated
Most styes resolve on their own within a week or so. The standard recommendation is warm compresses, held against the closed eyelid for five to ten minutes, two or three times a day. The heat helps the blocked gland open and drain. Resist the urge to squeeze or pop a stye, which can spread the infection deeper.
Topical antibiotic drops or ointments are generally ineffective for styes because they don’t penetrate well into the blocked gland. If a stye doesn’t improve with warm compresses, a doctor may lance it with a small blade to release the pus. Internal styes are more likely to need oral antibiotics along with drainage.
When a Stye Becomes Something Worse
In rare cases, the infection from a stye spreads into the surrounding tissue, causing a condition called preseptal cellulitis. The skin around the eye becomes swollen, warm, red, and tender, sometimes so puffy that it’s hard to open the lid. A fever may develop. The important distinction is that vision and eye movement stay normal, and the eyeball itself doesn’t bulge forward. If it did, that would suggest a deeper orbital infection requiring emergency treatment.
Preseptal cellulitis from a stye is treated with oral antibiotics. It’s uncommon, but it’s the main reason to pay attention if a stye keeps getting worse after several days instead of improving, or if redness and swelling start spreading beyond the bump itself.
Reducing Your Risk
Since S. aureus is the culprit, prevention comes down to keeping the bacteria away from your eyelid glands. Wash your hands before touching your eyes or handling contact lenses. Clean your lids regularly if you’re prone to styes, using a gentle cleanser or warm water on a clean washcloth. Replace eye makeup on schedule and never share it. If you wear contacts, follow your cleaning routine strictly, and avoid sleeping in lenses that aren’t designed for overnight wear.
People who get styes repeatedly sometimes have an underlying issue like chronic eyelid inflammation or rosacea that keeps the glands irritated and vulnerable. Treating that root cause, often with daily lid hygiene and warm compresses as a preventive habit, can break the cycle.