A stye contains trapped oil, bacteria, and pus. It forms when one of the tiny oil-producing glands in your eyelid becomes blocked, allowing bacteria to multiply inside. The resulting pocket of infection is what creates that painful, swollen bump at the edge of your eye.
What’s Actually Inside a Stye
Your eyelids contain dozens of small glands that produce oily secretions. These oils normally coat the surface of your eye and keep your tears from evaporating too quickly. When one of these glands gets clogged, the oil builds up behind the blockage with nowhere to go. Bacteria, almost always Staphylococcus aureus (the same staph bacteria responsible for many common skin infections), take advantage of the stagnant oil and begin multiplying inside the gland.
Your immune system responds by sending white blood cells to fight the infection. The combination of trapped oil, dead bacteria, and dead white blood cells forms pus, which is the yellowish material you may eventually see at the tip of a stye. This is the same process behind any small skin abscess, just happening in the delicate tissue of your eyelid.
Adults get styes more often than children because adult oil glands produce thicker secretions, making blockages more likely.
External vs. Internal Styes
The specific gland involved determines what type of stye you have and where the contents sit. External styes form at the base of an eyelash, originating from the small oil or sweat glands right at the lash line. These are the most visible type and tend to develop a white or yellow head on the outside of the eyelid, similar to a pimple.
Internal styes form deeper inside the eyelid, in larger oil glands embedded in the firm tissue of the lid itself. Because they’re farther from the surface, internal styes may not develop a visible head. Instead, you’ll feel a tender lump under the skin of the eyelid, and the swelling often points inward toward the eye rather than outward. The contents are the same (oil, bacteria, pus), but the location makes internal styes slower to drain on their own.
How a Stye Drains
Most styes follow a predictable course. Over several days, pressure builds inside the blocked gland until the pus collection develops a visible “point” or head. This head typically bursts on its own within three to four days, releasing a small amount of pus. Once it drains, the pain and swelling resolve quickly. Some styes take longer, lasting up to one to two weeks before fully clearing.
Warm compresses speed this process along. Soaking a clean washcloth in warm water and holding it gently against the affected eye for five minutes, several times a day, helps soften the hardened oil plug and encourages the gland to open. The warmth essentially liquefies the thickened oil so it can flow out naturally. Resist the urge to squeeze or pop a stye. Forcing the contents out can push bacteria deeper into the eyelid tissue or spread infection to surrounding areas.
How a Stye Differs From a Chalazion
A chalazion looks similar to a stye but contains different material. While a stye is an active bacterial infection filled with pus, a chalazion is a clogged oil gland without infection. The trapped oil triggers inflammation, so the bump swells and becomes firm, but there’s no bacterial invasion and no pus inside. Chalazia are typically painless, harder to the touch, and sit farther from the eyelid margin than styes.
Sometimes a stye that doesn’t fully drain transitions into a chalazion. The infection resolves, but the gland remains blocked, leaving behind a firm, painless lump of thickened oil and inflammatory tissue. This can persist for weeks or months if the blockage doesn’t clear.
When the Infection Spreads
In rare cases, the bacteria inside a stye can escape the gland and infect the surrounding eyelid tissue, a condition called preseptal cellulitis. Signs include widespread swelling, warmth, tenderness, and redness across the eyelid, sometimes severe enough that you can’t easily open your eye. A fever may develop. The key distinction is that your vision and eye movement remain normal, since the infection stays in front of the bony wall that protects the eye socket. If swelling spreads rapidly, vision changes, or you develop pain with eye movement, those are signs the infection may have moved deeper and needs urgent evaluation.