Sinusitis develops when your sinuses, the air-filled spaces behind your forehead, cheeks, and eyes, become inflamed and swollen enough to trap mucus that would normally drain freely. The trigger is almost always something that blocks the narrow drainage openings of the sinuses or damages the tiny hair-like structures (cilia) that sweep mucus out. Once mucus pools inside a sealed-off sinus, bacteria, viruses, or fungi can thrive, turning inflammation into a full infection. About 29 million American adults are diagnosed with sinusitis each year.
The Basic Mechanism: Blocked Drainage
Each sinus connects to your nasal cavity through a small opening called an ostium. These openings are only a few millimeters wide. When the tissue lining those openings swells, whether from a cold, allergies, or irritants, the passage narrows or closes entirely. Mucus that your sinuses constantly produce has nowhere to go, and stagnant mucus becomes a breeding ground for infection.
At the same time, the cilia lining your sinuses can be damaged or slowed by viruses, smoke, or dry air. Healthy cilia beat in coordinated waves to push mucus toward the drainage openings. When they stop working properly, mucus thickens and accumulates even if the openings aren’t completely blocked. This combination of swelling and impaired mucus clearance is the core of how sinusitis starts, regardless of the specific cause.
Viral Infections: The Most Common Cause
The overwhelming majority of sinus infections begin as a common cold. Over 200 different viruses can trigger acute viral sinusitis. Rhinoviruses account for 25% to 80% of cases, followed by coronaviruses (10% to 20%), influenza viruses (10% to 15%), and adenoviruses (about 5%). In 20% to 30% of cases, no specific virus is ever identified.
A typical cold inflames the nasal lining, which extends into the sinuses. Most of the time, this viral inflammation resolves on its own within 7 to 10 days. Sinusitis is really what happens when the inflammation lingers or is severe enough to fully block drainage. You can’t “catch” sinusitis directly from someone, but you can catch the virus that causes it. These viruses spread through respiratory droplets when an infected person coughs, sneezes, or talks, and through contaminated surfaces.
When Bacteria Take Over
Bacterial sinusitis is almost always a secondary infection. It develops when a viral cold or allergic episode has already created the conditions for bacteria to grow: blocked drainage, stagnant mucus, and damaged tissue. Only a small fraction of sinus infections are bacterial. The classic sign is symptoms that worsen after initially improving, or symptoms that persist beyond 10 days without getting better.
The two bacteria most commonly responsible are Streptococcus pneumoniae and Haemophilus influenzae. These organisms are often already present in your nose and throat in small numbers. They become a problem only when trapped mucus gives them an environment to multiply unchecked. This is why antibiotics are rarely helpful for most sinus infections: the initial cause is viral, and the body can usually clear the infection once drainage is restored.
Allergies and Chronic Inflammation
Allergic rhinitis is one of the most significant risk factors for recurrent and chronic sinusitis. When you inhale an allergen like pollen, dust mites, or pet dander, immune cells in your nasal lining release histamine and other inflammatory chemicals within minutes. These chemicals cause the familiar allergy symptoms: sneezing, runny nose, and congestion. But the swelling doesn’t stop at the nose. The same inflammation can spread to the sinus linings, narrowing or blocking the drainage openings.
What makes allergies particularly problematic is that they sustain inflammation over long periods. Inflammatory cells enter the bloodstream and travel to tissues beyond the nose, including the sinuses, where they perpetuate swelling. Over time, this chronic inflammation can lead to thickened sinus tissue, nasal polyps, and a cycle of repeated infections. People with year-round allergies face a higher risk of developing chronic sinusitis, defined as symptoms lasting 12 weeks or longer, compared to those with only seasonal exposure.
Structural Problems That Trap Mucus
Some people are anatomically prone to sinusitis. A deviated septum, the wall between the two sides of your nose, can lean far enough to one side to physically narrow or block a sinus opening. Nasal polyps, soft, painless growths that develop on the lining of the nasal passages, can do the same thing. When polyps grow large enough, they obstruct airflow and sinus drainage, making it difficult to breathe through your nose and creating the stagnant conditions infections need.
These structural issues don’t cause infections on their own, but they lower the threshold. Someone with a significantly deviated septum might develop sinusitis from a mild cold that would cause no problems in someone with wide-open drainage pathways. This explains why some people seem to get sinus infections repeatedly while others rarely do, even when exposed to the same viruses.
Fungal Sinusitis
Fungi are everywhere in the environment, and tiny spores enter your sinuses every time you breathe. For most people, this is harmless. Fungal sinusitis becomes a concern in two situations. Allergic fungal sinusitis occurs when your immune system overreacts to fungal spores in the sinuses, producing thick mucus and sometimes nasal polyps, even in otherwise healthy people. Invasive fungal sinusitis is far more serious and occurs almost exclusively in people with weakened immune systems, such as those undergoing chemotherapy or organ transplant recipients. In invasive cases, the fungus can spread beyond the sinuses to the eyes and brain, making it a medical emergency.
Environmental and Lifestyle Risk Factors
Cigarette smoke, including secondhand exposure, directly impairs the sinuses’ ability to clear mucus. Smoke damages the cilia, increases bacterial attachment to the nasal lining, and disrupts the cells that make up the sinus tissue. People regularly exposed to secondhand smoke have a measurably higher risk of developing chronic sinusitis.
Other environmental factors that raise your risk include dry indoor air (especially during winter heating season), air pollution, chlorinated pool water, and frequent flying. Anything that dries, irritates, or inflames the nasal lining can set the stage for blocked drainage. Swimming and diving can also force contaminated water into the sinuses. Spending time in daycare centers or crowded workplaces increases exposure to the respiratory viruses that start the process.
Acute, Subacute, and Chronic: The Timeline
How you got sinusitis often determines how long it lasts. Acute sinusitis, the most common type, produces symptoms for less than four weeks and typically resolves with basic treatment or on its own. Subacute sinusitis lasts four to eight weeks, often because the initial infection didn’t fully clear or the underlying inflammation persists. Chronic sinusitis means symptoms have continued for 12 weeks or longer, and it usually involves a different set of causes: ongoing allergies, structural problems, immune deficiencies, or repeated bacterial infections that never fully resolve.
The transition from acute to chronic is not simply a matter of time passing. Chronic sinusitis involves lasting changes to the sinus tissue itself, including thickening, polyp formation, and persistent low-grade inflammation that can continue even after an active infection clears. This is why chronic sinusitis often requires a different approach, one focused on reducing inflammation and restoring drainage rather than just fighting infection.