A sty forms when bacteria, almost always Staphylococcus aureus, infect one of the tiny oil-producing glands along your eyelid margin. About 90% to 95% of styes trace back to this single bacterium, which often already lives on your skin and lashes in small numbers. The infection starts when a gland gets blocked, trapping oily secretions inside and giving bacteria a rich environment to multiply.
What Happens Inside Your Eyelid
Your eyelids contain dozens of small glands that release oils into your tear film, keeping your eyes lubricated and protected. When one of these glands gets clogged by debris, dead skin cells, dried oil, or bacterial buildup, the blockage traps the gland’s oily secretions. Bacteria that were harmlessly sitting on the skin surface now have a warm, nutrient-rich pocket to colonize.
Once bacteria begin multiplying inside the blocked gland, your immune system responds by sending white blood cells to the area. This creates the familiar signs of a sty: a red, swollen, tender bump that may develop a visible whitish or yellowish head as pus collects. The whole process, from initial blockage to a full-blown painful bump, typically unfolds over a day or two.
The bacteria also produce enzymes that break down fats in the gland, releasing fatty acids and cholesterol. Cholesterol actually stimulates further bacterial growth, which is part of why a sty can feel like it gets worse before it gets better. The released fatty acids also irritate surrounding tissue, adding to the inflammation and tenderness.
External vs. Internal Styes
Not all styes look or feel the same, because your eyelid has different types of glands. An external sty affects the smaller oil glands near the base of your eyelashes. These tend to form right at the lash line and are usually visible as a small pimple-like bump on the outside of the lid.
An internal sty develops in the larger oil glands embedded deeper within the eyelid itself (the meibomian glands). These are often more painful and may not be visible from the outside at first. You might feel a deep ache or notice swelling across a wider area of the lid before a distinct bump becomes apparent. Both types involve the same basic process of blockage followed by infection, but internal styes can take longer to resolve because the infection sits deeper in the tissue.
The Most Common Causes
Touching your eyes with unwashed hands is the single most straightforward way bacteria reach your eyelid glands. Every time you rub your eyes, adjust your lashes, or wipe away a tear with dirty fingers, you’re potentially introducing S. aureus or other bacteria directly to the gland openings along your lid margin.
Old or shared eye makeup is another major source. A study analyzing used mascara samples found S. aureus in 79% of them. These products sit at warm room temperature, get repeatedly exposed to the bacteria on your skin, and are applied directly to the lash line, right where gland openings are most vulnerable. Using expired cosmetics or sharing eye makeup with others significantly increases your exposure.
Contact lens habits also play a role. Handling lenses with unwashed hands, sleeping in contacts, topping off old disinfecting solution instead of replacing it, rinsing lenses in tap water, and wearing lenses past their recommended replacement schedule all create opportunities for bacteria to reach the eye. Even household tap water, while safe to drink, is not sterile and contains microorganisms that can transfer to the eye area through contaminated lenses or lens cases.
Conditions That Make Styes More Likely
Some people get styes repeatedly, and that often points to an underlying condition rather than just bad luck with hygiene. Blepharitis, a chronic inflammation of the eyelid margins, keeps the gland openings irritated and partially blocked, making them easy targets for bacterial overgrowth. If your eyelids frequently feel crusty, itchy, or look red along the lash line, blepharitis may be the reason styes keep returning.
Meibomian gland dysfunction (MGD) is closely linked to recurring styes. In MGD, the deeper eyelid glands don’t function properly. Their secretions become thickened and waxy, plugging the gland openings and setting the stage for repeated infections. Chronic styes are actually one of the hallmark signs that MGD may be present.
Rosacea, particularly when it affects the eyes, is another significant risk factor. Ocular rosacea triggers an overactive immune response that creates chronic inflammation on the eye’s surface. This inflammation disrupts the normal balance of microbes living around the eye, damages the meibomian glands over time, and can alter the composition of tear film. Tiny Demodex mites, which live in hair follicles along the lash line, tend to proliferate in people with ocular rosacea and carry bacteria that further fuel the inflammatory cycle.
How a Sty Typically Progresses
Most styes follow a predictable pattern. You’ll first notice tenderness or a slightly swollen spot on your eyelid. Over the next day or so, the area becomes redder and more swollen, and a distinct bump forms. The bump may develop a visible pus-filled head, similar to a pimple. Your eye might water more than usual, and the lid can feel heavy or irritated.
The majority of styes drain on their own within 7 to 10 days. Applying a clean, warm compress for 10 to 15 minutes several times a day helps soften the blocked material and encourages the gland to open and drain. Resist the urge to squeeze or pop a sty. Squeezing can push the infection deeper into the tissue or spread bacteria to neighboring glands.
If a sty doesn’t drain and the inflammation fades but a firm, painless lump remains, it has likely turned into a chalazion. A chalazion is no longer an active infection but rather a pocket of trapped, hardened gland secretions surrounded by inflammatory tissue. Chalazions can persist for weeks or months and sometimes need minor in-office treatment to resolve.
Reducing Your Risk
Keeping your hands away from your eyes is the simplest prevention measure, and washing your hands thoroughly before any contact with your eye area (inserting contacts, applying makeup, removing a lash) makes a meaningful difference. Replace mascara and liquid eyeliner every three months, and never share eye cosmetics.
If you wear contact lenses, follow replacement schedules strictly, always use fresh disinfecting solution rather than topping off old solution, and never store or rinse lenses in tap water. Wash your hands before both inserting and removing lenses.
For people prone to recurring styes, a nightly lid hygiene routine can help keep gland openings clear. This typically involves a warm compress held against closed eyelids for several minutes, followed by gentle cleaning of the lash line with a diluted baby shampoo solution or a commercially available lid scrub. This softens any thickened oils in the glands and removes the debris and bacterial buildup that lead to blockages. If styes keep coming back despite good hygiene, the pattern is worth mentioning to an eye care provider, since it may signal blepharitis, MGD, or ocular rosacea that benefits from targeted treatment.