Most sinus infections start with a common cold. A virus inflames the lining of your sinuses, the air-filled cavities behind your forehead, cheeks, and eyes. That swelling traps mucus in spaces that normally drain freely, creating the pressure, congestion, and pain you recognize as a sinus infection. Only about 0.5% to 2% of these viral infections progress to a bacterial sinus infection, meaning the vast majority resolve on their own without antibiotics.
Viruses Are the Most Common Cause
The same viruses that give you a cold are responsible for most sinus infections. Rhinovirus is the leading culprit, causing more upper respiratory infections than any other pathogen. Respiratory syncytial virus (RSV) is the second most commonly identified virus in sinus infections, followed by influenza A and B and common human coronaviruses.
When one of these viruses reaches your nasal passages, it triggers an immune response that swells the tissue lining your sinuses. That swelling narrows or blocks the small openings (called ostia) that connect each sinus to your nasal cavity. Mucus that would normally flow out gets trapped instead. The warm, moist, stagnant environment is what produces the familiar symptoms: facial pressure, thick nasal discharge, and congestion that can last 7 to 10 days. In most cases, the inflammation clears as your immune system fights off the virus, and your sinuses begin draining normally again.
When Bacteria Take Over
A small percentage of viral sinus infections create conditions ripe for bacteria to multiply. If mucus sits trapped in a sinus cavity long enough, bacteria already present in your nasal passages can overgrow in that stagnant fluid. The three bacteria most commonly involved are Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis.
Bacterial sinus infections tend to follow a recognizable pattern. You start feeling better from a cold, then your symptoms worsen again, or your symptoms persist well beyond the 10-day window typical for a viral infection. The discharge often becomes thicker and more discolored, and facial pain may concentrate over a specific sinus. Even so, distinguishing bacterial from viral sinusitis based on symptoms alone is unreliable. Studies show that imaging findings like fluid levels and even the presence of purulent discharge don’t accurately predict which patients actually have bacterial growth. Placebo-controlled trials find that about 80% of people with acute sinus infections improve without antibiotics, compared to 90% with them, reinforcing that most cases don’t require treatment for bacteria.
How Allergies Set the Stage
Allergic rhinitis, the sneezing and congestion triggered by pollen, dust mites, mold, or pet dander, is one of the most common contributors to sinus infections. When your immune system reacts to an allergen, blood flow increases in the nasal lining, and the tissue swells. This swelling can narrow or block sinus drainage openings in the same way a viral infection does, trapping mucus and creating favorable conditions for bacterial growth.
The connection goes beyond a single episode. People with persistent allergies experience chronic low-grade inflammation in their nasal passages, which keeps the sinus linings irritated and drainage impaired over weeks or months. This is why sinus infections often cluster during allergy season and why treating the underlying allergy, through antihistamines or nasal corticosteroid sprays, can reduce the frequency of infections.
Structural Problems That Block Drainage
Your sinus anatomy plays a bigger role than most people realize. A deviated septum, where the wall between your nostrils leans to one side, can physically restrict airflow and impair drainage on the narrower side. Nasal polyps, soft growths that develop on the lining of the nasal passages or sinuses, can partially or fully obstruct sinus openings. Enlarged turbinates, the bony structures inside your nose that warm and humidify air, can also swell enough to block drainage.
These structural issues don’t cause infections directly, but they create a persistent bottleneck. Every time you get a cold or an allergy flare, your already-narrow passages are more likely to close off completely. This is why some people get sinus infections repeatedly while others rarely do, and it’s one reason doctors investigate anatomy in patients with chronic or recurrent infections.
Smoking and Air Quality
Your sinuses rely on a self-cleaning system: tiny hair-like structures called cilia beat in coordinated waves to push mucus toward drainage openings. Tobacco smoke disrupts this process at multiple levels. Lab studies on human sinus tissue show that cigarette smoke extract slows ciliary beating, impairs the chemical transport that keeps the mucus layer fluid, and at higher concentrations damages the development of new cilia altogether. The result is sluggish mucus clearance, which means irritants and pathogens sit in the sinuses longer.
Air pollution and particulate matter can cause similar problems, irritating the nasal lining and impairing the same mucus-clearing system. People who live in areas with heavy air pollution or who work around dust, chemical fumes, or industrial irritants face a higher risk of both acute and chronic sinus problems.
Fungal Sinus Infections
Fungi cause a small but important subset of sinus infections. Allergic fungal sinusitis is the most common type and occurs in people with functioning immune systems. It happens when the immune system overreacts to fungal spores inhaled from the environment, producing thick, dark, rubbery mucus that fills the sinuses. People affected tend to be younger, often in their 20s and 30s, and frequently have nasal polyps and asthma. The fungi most commonly recovered are Bipolaris and Curvularia, though Aspergillus was historically considered the primary cause.
Invasive fungal sinusitis is far more dangerous and far less common. It occurs almost exclusively in people with severely weakened immune systems, such as uncontrolled diabetes or advanced immunodeficiency. Instead of staying confined to the sinus lining, the fungus invades blood vessels, nerves, and bone. This type carries a mortality rate around 50% and requires aggressive treatment.
Chronic Sinusitis and Why It Persists
When sinus inflammation lasts 12 weeks or longer, it’s classified as chronic rhinosinusitis. The causes overlap with acute sinusitis but involve additional factors that keep the cycle going. Chronic sinusitis is now understood as a multifactorial disease, driven by a combination of immune dysfunction, persistent inflammation, allergies, asthma, and structural factors rather than a single pathogen.
One major contributor to persistent infections is biofilm formation. Bacteria like Staphylococcus aureus and Pseudomonas aeruginosa can form protective communities on sinus tissue, encasing themselves in a sticky matrix that shields them from both your immune system and antibiotics. Patients who develop these biofilms tend to have worse outcomes after treatment and higher rates of relapse. Staphylococcus aureus biofilms in particular are linked to the most severe cases and are more common in people who also have asthma.
Conditions that affect mucus composition or ciliary function also contribute. People with cystic fibrosis produce abnormally thick mucus that clogs sinus passages and provides a growth medium for bacteria. Primary ciliary dyskinesia, a genetic condition where cilia don’t beat properly, leads to chronic mucus stagnation. In both conditions, sinus infections are a near-constant problem rather than an occasional one.
Asthma and chronic sinusitis frequently coexist. The relationship runs in both directions: sinus inflammation can worsen asthma, and the type of immune response seen in asthma (eosinophilic, or type 2 inflammation) is the same pattern found in the most severe and recurrence-prone forms of chronic sinusitis, particularly those involving nasal polyps.