Most sinus infections start with a common cold. A virus inflames the lining of your nasal passages, that tissue swells, and the tiny drainage openings of your sinuses get blocked. Mucus that normally flows out freely gets trapped, creating a warm, stagnant environment where bacteria can multiply. Between 90% and 98% of all sinus infections are viral, meaning the cold itself is the sinus infection. Only 2% to 10% progress to a secondary bacterial infection.
What Happens Inside Your Sinuses
Your sinuses are air-filled cavities behind your forehead, cheeks, and eyes. They’re lined with a thin layer of mucus that’s constantly swept toward the back of your throat by microscopic hair-like structures called cilia, which beat about 8 to 15 times per second and move mucus at roughly 6 millimeters per minute. This self-cleaning system works well as long as two things stay true: the mucus stays thin enough to flow, and the drainage openings stay open.
Those drainage openings are only 1 to 3 millimeters wide. The frontal sinuses (behind your forehead), the maxillary sinuses (behind your cheeks), and the anterior ethmoid sinuses (between your eyes) all share a common drainage pathway called the ostiomeatal complex. Because they funnel through the same narrow area, even a small amount of swelling can block drainage from multiple sinuses at once. That’s why a sinus infection often hits several areas of your face simultaneously.
Once blocked, the trapped mucus loses oxygen, drops in pH, and becomes a breeding ground for bacteria that already live harmlessly in your nasal passages. Your immune system responds with more inflammation, which causes more swelling, which traps more mucus. This feedback loop is why sinus infections can feel like they’re getting worse for days before they get better.
The Common Cold Is the Most Frequent Trigger
The typical path to a sinus infection looks like this: you catch a cold, your nasal lining swells, your sinuses can’t drain, and the stagnant mucus either causes prolonged viral symptoms or, less commonly, becomes secondarily infected with bacteria. Most colds resolve in 7 to 10 days. If your symptoms persist beyond 10 days without improving, include a fever of 102°F or higher with facial pain and nasal discharge lasting 3 to 4 days, or seem to get better after a week only to suddenly worsen again, a bacterial infection is the likely cause.
That “got better then got worse” pattern is a useful signal. It suggests your immune system was handling the initial viral infection, but bacteria took hold in the lingering mucus before your sinuses fully cleared.
Allergies Can Set the Stage
If you have seasonal or year-round allergies, your risk of sinus infections goes up significantly. Allergic reactions cause the blood vessels in your nasal tissue to dilate, and because nasal tissue is highly vascular, even modest vascular changes can produce significant obstruction. The swelling impairs mucociliary clearance, slowing the movement of mucus and giving bacteria more time to colonize.
The connection between allergies and sinus infections is partly anatomical. The nasal turbinates (ridges of tissue inside your nose) and the ethmoid sinuses share blood supply, nerve pathways, and direct anatomic connections. When your turbinates swell from an allergic reaction, the inflammation extends into the adjacent sinus drainage pathways. This is why people with poorly controlled allergies often deal with repeated sinus infections, even outside of cold season.
Structural Issues That Block Drainage
Some people are simply built in a way that makes sinus infections more likely. A deviated septum, where the wall between your nostrils is shifted to one side, can narrow the ostiomeatal complex and make it more sensitive to obstruction from even mild swelling. Other anatomical variations, like unusually curved middle turbinates or small air cells that protrude into the drainage pathway, have the same effect.
Nasal polyps are another common culprit. These soft, painless growths develop on the lining of the nasal passages or sinuses, and when they grow large enough, they physically block drainage. People with asthma and allergic rhinitis are more likely to develop polyps, which partly explains why these conditions tend to cluster together with chronic sinus problems.
Smoking and Air Pollution
Cigarette smoke and airborne particulate matter damage the protective barrier lining your nasal passages. Research from Johns Hopkins has shown that chronic exposure to air pollution breaks down the proteins that hold nasal lining cells together, creating gaps that allow allergens, viruses, and bacteria easier access to the underlying tissue. Smoke also paralyzes cilia, slowing or stopping the mucus-clearing mechanism your sinuses depend on. If you smoke or live in an area with high air pollution, your sinuses are working with a compromised defense system.
Swimming and Diving
Water exposure introduces a less obvious pathway to sinus infections. Diving creates pressure changes that your sinuses need to equalize through their small openings. If those openings are even partially blocked from a mild cold or allergies, the pressure difference can cause the blood vessels in the sinus lining to engorge and burst. The collected blood then acts as a growth medium for bacteria, potentially leading to infection. Even recreational swimming in contaminated water can introduce bacteria directly into the nasal passages.
Fungal Sinus Infections
Not all sinus infections are caused by viruses or bacteria. Fungi that exist naturally in the air can cause sinus infections, particularly in people with allergies or weakened immune systems. The most common type is allergic fungal sinusitis, where your body mounts an allergic reaction to fungi in the nose, filling the sinuses with thick mucus and sometimes producing nasal polyps. Another form, called a fungal ball, occurs when fungi accumulate in a sinus and form a dense clump.
Invasive fungal sinus infections are rare but serious. They primarily affect people with compromised immune systems, including those with unmanaged diabetes, HIV, cancer, or anyone taking immunosuppressive medications. In these cases, fungi can destroy blood vessels and tissue in the nose and potentially spread to the eyes or brain.
When a Sinus Infection Spreads
The vast majority of sinus infections resolve on their own or with treatment, but the proximity of the sinuses to the eyes and brain means complications are possible. The ethmoid sinuses sit right next to the eye sockets. An infection that extends beyond the sinus wall can cause the eye to swell shut, bulge forward, and develop blurred vision or limited movement. Sharp eye pain with a sinus infection is a warning sign that needs prompt attention.
Intracranial complications are rarer but can present subtly, sometimes with only a persistent headache and low-grade fever. These complications are more likely with frontal sinus infections, where the back wall of the sinus is also the front wall of the brain cavity. In practice, these serious complications are uncommon, but they’re the reason sinus infections that produce severe headaches, visual changes, or high fevers warrant urgent evaluation.