Most sinus infections in adults start with a common cold. Viruses cause the vast majority of cases, swelling the tissue that lines your sinuses and trapping mucus in spaces that normally drain freely. From there, bacteria can move in, allergies can set the stage, or structural issues in your nose can keep the cycle going. Understanding which cause is behind your symptoms matters because it shapes what will actually help.
How a Sinus Infection Develops
Your sinuses are air-filled cavities behind your forehead, cheeks, and eyes, all connected to your nasal passages through small openings. These openings allow mucus to drain out continuously, carrying away dust, bacteria, and other debris. The system works well when everything is flowing.
A sinus infection starts when something blocks those drainage pathways. Swollen tissue, excess mucus, or a physical obstruction traps fluid inside the sinus cavity. That warm, moist, sealed-off environment becomes a breeding ground for bacteria or, less commonly, fungi. The longer the blockage lasts, the more likely infection takes hold. This basic mechanism, blocked drainage followed by microbial overgrowth, applies regardless of whether the original trigger is a virus, an allergy, or a structural problem in the nose.
Viral Infections: The Most Common Trigger
The typical cold virus inflames the lining of your nasal passages and sinuses, which swells enough to partially or fully block sinus drainage. This is why sinus pressure and congestion are standard cold symptoms. In most people, the swelling resolves on its own within 7 to 10 days and the sinuses return to normal.
The trouble comes when congestion lingers. Bacteria that normally live harmlessly in the nose can multiply in the stagnant mucus and cause a secondary bacterial infection. Current clinical guidelines from the American Academy of Otolaryngology consider bacterial sinusitis likely when symptoms don’t improve at all within 10 days of getting sick, or when they start to get better and then worsen again within that 10-day window. That “double worsening” pattern, feeling better then suddenly worse, is one of the most reliable signs that a viral infection has progressed to a bacterial one.
Allergies and Chronic Inflammation
Allergic rhinitis (hay fever) is one of the strongest predisposing factors for sinus infections. When you’re allergic to pollen, dust mites, mold, or pet dander, your immune system triggers inflammation in the nasal lining every time you’re exposed. That inflammation narrows the sinus drainage pathways in the same way a cold virus does, creating the same conditions for bacterial overgrowth.
The connection goes beyond a single episode. Chronic allergic inflammation can keep the sinus openings partially obstructed for weeks or months at a time. Research from Stony Brook Medicine describes this as a cycle: allergies cause swelling and obstruction, which leads to bacterial colonization, which can progress to acute, recurrent, or chronic sinusitis. People with poorly controlled allergies often find themselves dealing with repeated sinus infections because the underlying inflammation never fully resolves between episodes.
Structural Problems in the Nose
Some people are more prone to sinus infections because of the physical shape of their nasal passages. A deviated septum, where the wall between the nostrils sits off-center rather than in the middle, can make one or both nasal passages significantly smaller. That reduced airflow and drainage capacity means it takes less swelling to fully block a sinus opening.
Nasal polyps are another common structural cause. These soft, noncancerous growths develop in the nose and sinuses after prolonged inflammation, typically lasting 12 weeks or longer. As polyps grow and accumulate, they physically obstruct airflow and mucus drainage, creating persistent stuffiness that feeds more inflammation. The exact reason some people develop polyps and others don’t isn’t fully understood, but conditions involving long-term nasal inflammation, including allergies and asthma, increase the risk. A deviated septum can also contribute to polyp formation by promoting the kind of chronic blockage and inflammation that triggers their growth.
Dental Infections
This is a cause many people don’t expect. The roots of your upper back teeth sit very close to the floor of your maxillary sinuses, the large sinuses behind your cheekbones. An infected tooth, a dental abscess, or complications from dental procedures like extractions or implants can spread infection directly into the sinus above.
A 2023 meta-analysis in the Journal of Endodontics found that roughly half of all maxillary sinus infections may originate from a dental source. That’s a striking number, and it means a sinus infection that keeps coming back on one side of your face, especially with tooth pain or a recent dental history, could have a cause that no amount of decongestants will fix. Treating the dental problem is what resolves the sinus infection in these cases.
Environmental Irritants
Cigarette smoke, air pollution, and chemical fumes can all damage the lining of your sinuses in ways that make infections more likely. Research from Johns Hopkins Medicine showed that chronic exposure to airborne particulate matter breaks down the proteins that hold the cells of the sinus lining together. When those cellular connections weaken, the barrier becomes more permeable to allergens, viruses, and bacteria that would normally be kept out.
The damage also includes a thicker, more inflamed sinus lining, which further narrows drainage pathways. Smokers and people living in areas with high air pollution carry a consistently higher risk of chronic sinusitis. Secondhand smoke has similar effects, particularly in shared indoor spaces with poor ventilation.
Fungal Sinus Infections
Fungi are a less common but important cause of sinusitis, and they behave very differently depending on the type. Most fungal sinus infections are noninvasive, meaning they stay within the nose and sinus areas. Allergic fungal sinusitis is the most common form: your immune system overreacts to fungi (molds and yeasts) that enter the nose through normal breathing, filling the sinuses with thick mucus. People with asthma or hay fever are more likely to develop this type, and nasal polyps often form alongside it.
Invasive fungal sinusitis is rare but far more serious. It occurs almost exclusively in people with severely weakened immune systems. In this form, fungi destroy blood vessels inside the nose, killing the surrounding tissue. It can spread rapidly to the eyes and brain and requires emergency treatment. The distinction between these two types matters enormously: one is a chronic nuisance, the other is a medical emergency.
Acute vs. Chronic: Why Duration Matters
Sinus infections are classified by how long they last, and the underlying causes tend to differ between short and long episodes. Acute sinusitis lasts less than 4 weeks and is most often triggered by a viral cold that may or may not progress to a bacterial infection. These typically resolve with time or, when bacterial, with a course of antibiotics.
Chronic sinusitis lasts 12 weeks or longer and is usually driven by something that won’t go away on its own: uncontrolled allergies, nasal polyps, a deviated septum, a hidden dental infection, or ongoing environmental exposure. Recurrent sinusitis, defined as four or more acute episodes per year, also points toward a persistent underlying cause rather than simple bad luck with colds. If your sinus infections keep coming back, identifying and addressing the specific trigger is more effective than treating each episode individually.
Immune System and Other Risk Factors
Anything that weakens your immune defenses can increase susceptibility to sinus infections. Diabetes, HIV, and immunosuppressive medications all raise the risk, as does simple fatigue and stress that impair day-to-day immune function. Conditions that affect mucus consistency, like cystic fibrosis, make it harder for the sinuses to clear themselves even when drainage pathways are open.
Swimming in chlorinated pools can irritate the nasal lining, and frequent air travel exposes you to dry cabin air and pressure changes that temporarily impair sinus drainage. Even prolonged use of nasal decongestant sprays, beyond 3 to 5 days, can cause rebound swelling that worsens congestion and sets the stage for infection.