Most sinus infections in adults start with a common cold. The virus causes swelling inside the nasal passages, which blocks the small openings that normally let your sinuses drain. Once mucus gets trapped, it creates a warm, moist environment where bacteria or fungi can multiply. But viruses aren’t the only trigger. Structural problems in your nose, dental infections, allergies, smoking, and even a weakened immune system can all set the stage for sinusitis.
Viruses Start the Majority of Cases
The overwhelming majority of acute sinus infections are viral, developing alongside or just after an ordinary cold. The same viruses that give you a stuffy nose and sore throat (rhinovirus, influenza, parainfluenza) inflame the lining of your sinuses. That inflammation narrows the drainage pathways, trapping mucus. In most people, this resolves on its own within seven to ten days.
A smaller percentage of cases turn bacterial. This happens when the stalled mucus becomes a breeding ground for bacteria. The most common culprits are Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus, and Moraxella catarrhalis. Three patterns suggest a bacterial infection rather than a lingering virus: symptoms that persist 10 days or longer without any improvement, a high fever (102°F or above) paired with thick nasal discharge and facial pain lasting three to four days, or symptoms that start to get better after four to seven days and then suddenly worsen again. That last pattern, sometimes called “double worsening,” is one of the clearest signals that bacteria have taken hold.
Allergies and Chronic Inflammation
Allergies are one of the most common non-infectious causes. When your immune system reacts to pollen, dust mites, mold, or pet dander, the tissue inside your nose swells in the same way it does during a cold. If that swelling is persistent or recurring, it repeatedly blocks sinus drainage and creates conditions ripe for infection. People with both asthma and allergies are at particularly high risk for chronic sinusitis, because the inflammatory process affects the entire airway.
Fungal sinus infections follow a related path. In people who are allergic to common airborne fungi (inhaled during normal breathing), the immune system overreacts and produces intense local swelling. That swelling blocks the sinus openings, which in turn traps more fungal particles inside, escalating the allergic response in a self-reinforcing cycle. Interestingly, fungi have been found in the sinuses of the vast majority of people undergoing surgery for chronic sinusitis, suggesting that for some individuals, even a normal amount of fungal exposure triggers a disproportionate immune response.
Structural Blockages in the Nose
Your sinuses drain through narrow openings, so even small structural problems can cause big issues. A deviated nasal septum (where the wall between your nostrils is shifted to one side), nasal polyps (soft, noncancerous growths on the sinus lining), and bone spurs inside the nose can all physically block those openings. When drainage is chronically impaired, mucus pools and infections recur. Surgery to correct a deviated septum or remove polyps can break this cycle for people who don’t respond to other treatments.
Dental Infections and the Maxillary Sinus
This is one of the most underrecognized causes of sinus infections. Your upper back teeth sit directly beneath your maxillary sinuses, separated by a thin layer of bone. When a tooth becomes infected, or when a dental procedure damages that barrier, bacteria can spread upward into the sinus. A 2023 systematic review found that roughly half of all maxillary sinus infections may originate from a dental source. If you have a sinus infection that keeps coming back on one side of your face, especially with upper tooth pain, a dental problem could be the underlying cause.
Smoking and Secondhand Smoke
Cigarette smoke, whether you inhale it directly or breathe it in secondhand, damages the tiny hair-like structures (cilia) that sweep mucus out of your sinuses. Without effective ciliary function, mucus stagnates, and infections follow. Research published in ENTtoday found that current or childhood exposure to secondhand smoke more than doubled the risk of chronic sinusitis symptoms. Among people who already had chronic sinusitis, those exposed to secondhand smoke reported significantly worse nasal obstruction and discharge than those without exposure. Smoking doesn’t just raise your risk of getting a sinus infection; it makes existing infections harder to clear and more severe.
Weakened Immune System
Conditions that suppress immune function, including HIV, diabetes, autoimmune disorders treated with immunosuppressive medications, and chemotherapy, make it harder for your body to fight off the bacteria and fungi that enter your sinuses every day. Genetic conditions like cystic fibrosis and primary ciliary dyskinesia are also important. Cystic fibrosis causes unusually thick mucus that clogs the sinuses, while primary ciliary dyskinesia means the cilia lining your sinuses don’t move properly. Both conditions lead to recurrent infections starting in childhood and continuing into adulthood.
Why Some Infections Become Chronic
Acute sinusitis lasts less than four weeks. Subacute cases linger four to eight weeks. When symptoms persist for 12 weeks or longer, it’s classified as chronic sinusitis. Recurrent sinusitis means three or more acute episodes per year. The shift from acute to chronic often involves more than just a persistent germ.
One major factor is bacterial biofilms. Instead of floating freely, bacteria can form dense, organized colonies on the sinus lining, encased in a protective matrix. These biofilms are over 1,000 times more resistant to antibiotics than the same bacteria in their free-floating form. They also resist the immune system’s normal clearance mechanisms. People with biofilm-positive chronic sinusitis tend to have more severe disease before surgery and are more likely to have persistent symptoms afterward. Biofilms dominated by Staphylococcus aureus are associated with the most treatment-resistant cases.
Chronic sinusitis usually results from a combination of factors working together: ongoing allergic inflammation that keeps the tissue swollen, a structural problem that impairs drainage, and biofilm-forming bacteria that resist treatment. Addressing just one of these factors often isn’t enough, which is why chronic sinusitis can be so frustrating to manage. Identifying which combination of causes applies to you is the key to finding a treatment approach that actually works.