Styes are caused by bacterial infections in the tiny oil glands or hair follicles along your eyelid margin. In 90 to 95% of cases, the culprit is Staphylococcus aureus, a common bacterium that lives on your skin and only causes problems when it gets trapped inside a blocked gland. The blockage creates a warm, sealed environment where bacteria multiply rapidly, triggering the red, painful bump you recognize as a stye.
How a Stye Forms Inside Your Eyelid
Your eyelids contain dozens of small glands that produce oils to keep your eyes lubricated. The glands of Zeis sit right at the base of your eyelashes and produce a waxy oil that coats each lash. When one of these glands gets clogged, bacteria already living on your skin can become trapped inside. The gland swells, fills with pus, and forms the classic painful bump on your eyelid’s edge. This is an external stye, the most common type.
Internal styes form deeper in the eyelid, in larger oil glands called meibomian glands. These glands line the inner surface of both your upper and lower lids and release oils that prevent your tear film from evaporating too quickly. An infected meibomian gland produces a bump that points inward toward your eye rather than outward. Internal styes tend to be more painful and can occasionally cause more significant inflammation, sometimes even triggering a low fever.
The Bacteria Behind It
Staphylococcus aureus dominates as the primary cause. This bacterium is part of your normal skin flora, colonizing the nose, skin, and eyelids of roughly 30% of people at any given time. It’s typically harmless on the skin’s surface but becomes a problem when it enters a blocked gland. Studies of eyelid infections have also identified Staphylococcus epidermidis (accounting for about 35% of bacterial isolates in one study) and, rarely, Pseudomonas aeruginosa. But S. aureus remains the overwhelming driver.
Common Triggers and Risk Factors
The bacterial infection is the direct cause, but several everyday habits and conditions make that infection more likely to happen.
Touching your eyes with unwashed hands is the most straightforward route. Your fingers transfer bacteria from surfaces, your phone, or your nose directly to your eyelid margin. This is especially relevant for contact lens wearers, who touch their eyes multiple times a day. Skipping handwashing before inserting or removing lenses significantly raises your risk.
Old or contaminated eye makeup is another major trigger. Mascara, eyeliner, and eyeshadow brushes accumulate bacteria over time. The general recommendation is to replace eye makeup every three months. Sharing makeup transfers someone else’s bacterial strains to your eyelids, and sleeping in makeup gives bacteria hours of uninterrupted contact with your gland openings.
Contact lens hygiene lapses create risk beyond just the hand-touching issue. Reusing old solution, sleeping in lenses not designed for overnight wear, or storing lenses in a dirty case all introduce bacteria close to the eyelid glands.
Skin Conditions That Make Styes Recurrent
If you get styes repeatedly, an underlying skin or eyelid condition is often involved. Blepharitis, a chronic inflammation of the eyelid margins, is one of the most common links. It develops when bacteria on the eyelids overpopulate or when the oil glands become chronically clogged. The result is crusty, irritated eyelid edges that are primed for stye formation. Blepharitis itself has several triggers, including seborrheic dermatitis (the same condition that causes dandruff on your scalp) and rosacea, an inflammatory skin condition that affects the face.
The connection works like a chain: rosacea or dandruff promotes blepharitis, blepharitis clogs and inflames the eyelid glands, and clogged glands become breeding grounds for stye-causing bacteria. If you notice flaky skin at the base of your eyelashes alongside recurring styes, blepharitis is a likely contributor worth addressing.
Stye vs. Chalazion
Not every eyelid bump is a stye. Chalazia (the plural of chalazion) look similar in the first day or two, starting as a red, swollen, painful spot on the eyelid. After that initial period, the two diverge. A stye stays painful, localizes to the eyelid margin near a lash, and often develops a visible yellowish pus-filled head. A chalazion migrates toward the center of the eyelid body, becomes a firm but painless nodule, and results from a blocked gland without active infection.
The distinction matters because chalazia don’t involve bacteria and won’t respond to antibiotics. They’re caused by trapped oil that hardens into a cyst-like lump. A stye can sometimes turn into a chalazion if the infection clears but the gland remains blocked.
How Long Styes Last
Most styes resolve on their own within one to two weeks. The bump gradually comes to a head, drains (often while you sleep), and heals without treatment. Warm compresses speed this process by softening the blocked oil and encouraging natural drainage. Hold a clean, warm washcloth against the closed eyelid for 10 to 15 minutes, several times a day. Resist the urge to squeeze or pop a stye, which can spread the infection deeper into the eyelid tissue.
If a stye doesn’t improve after two weeks of warm compresses, or if it grows larger rather than smaller, a doctor can drain it with a small incision or prescribe topical antibiotics.
When a Stye Becomes Serious
Styes are almost always harmless, but in rare cases the infection can spread beyond the gland into the surrounding eyelid tissue, a condition called preseptal cellulitis. Warning signs include swelling and redness that spread across the entire eyelid rather than staying localized, worsening pain despite home care, or fever that accompanies the eyelid symptoms.
The most urgent red flags are vision changes, pain when moving your eye, or a feeling of pressure behind the eye. These symptoms suggest the infection has moved past the eyelid into the deeper eye socket, which requires immediate medical treatment to prevent damage to the eye and surrounding structures.