Most sinus infections in adults start with a common cold. A virus inflames the lining of your sinuses, blocking the narrow drainage passages and trapping mucus inside. That warm, stagnant environment can then become a breeding ground for bacteria or, less commonly, fungi. Around 28.9 million American adults are diagnosed with sinusitis each year, making it one of the most common reasons people visit a doctor.
Viral Infections: The Most Common Starting Point
The vast majority of acute sinus infections are viral. The same viruses that cause colds, including rhinoviruses and influenza, infect the sinus lining and cause it to swell. This swelling narrows or completely blocks the tiny openings (called ostia) that normally let mucus drain from your sinuses into your nose. Once drainage stops, pressure builds, and you get that familiar aching fullness in your face. These viral infections typically resolve on their own within 7 to 10 days without any need for antibiotics.
When Bacteria Take Over
About 2% to 10% of viral sinus infections progress to a bacterial infection. This happens when trapped mucus sits long enough for bacteria to multiply inside the sinuses. The most common bacteria involved are Streptococcus pneumoniae, Haemophilus influenzae, Staphylococcus aureus, and Moraxella catarrhalis. H. influenzae alone accounts for 22% to 35% of bacterial sinus infections in adults.
There are a few patterns that distinguish a bacterial sinus infection from one that’s still viral. Symptoms lasting 10 days or more without improvement suggest bacteria have moved in. A high fever of 102°F or above with thick, discolored nasal discharge or severe facial pain in the first three days of illness is another sign. A third pattern, sometimes called “double sickening,” is when you start to feel better from a cold and then suddenly get worse again with new fever, increased discharge, or worsening headache.
Allergies and Chronic Inflammation
Allergic rhinitis (hay fever) is one of the biggest risk factors for recurring sinus infections. When your immune system overreacts to pollen, dust mites, mold, or pet dander, the lining of your nasal passages swells. That swelling obstructs sinus drainage the same way a viral infection does. Over time, chronic allergic inflammation can keep those drainage passages partially blocked, creating conditions where bacteria colonize the sinuses again and again.
This is why people with poorly controlled allergies often find themselves dealing with sinus infections multiple times a year. Managing the underlying allergy, whether through avoiding triggers or treating the inflammation directly, can break the cycle.
Structural Problems in the Nose
The physical shape of your nasal passages matters. A deviated septum, where the wall between your nostrils is significantly off-center, can narrow one side enough to impair drainage. Nasal polyps, which are soft, painless growths on the sinus lining, can do the same thing. When polyps grow large enough, they physically block the nasal passages and sinuses, leading to repeated infections. Other structural issues like a narrow sinus opening or swollen turbinates (the ridges inside your nose that warm and humidify air) can also contribute.
These problems don’t cause infections by themselves. They create the conditions for infection by preventing mucus from draining normally. If you keep getting sinus infections despite treating allergies and avoiding other triggers, a structural issue may be part of the picture.
Air Pollution and Cigarette Smoke
What you breathe plays a significant role. Research from Johns Hopkins Medicine found that long-term exposure to fine particulate air pollution (tiny particles smaller than 2.5 micrometers, about 30 times smaller than the width of a human hair) significantly increases the risk of chronic sinusitis. Exposure over 60 months was associated with roughly one-and-a-half times the risk of developing chronic sinus disease, and nearly five times the risk of severe inflammation across all four sinus pairs.
These particles come from vehicle exhaust, industrial emissions, wildfire smoke, dust, and soot. They irritate and inflame the sinus lining in a way that mirrors what infections and allergies do. Cigarette smoke, whether firsthand or secondhand, has the same effect. It paralyzes the tiny hair-like structures (cilia) that sweep mucus out of your sinuses, letting it pool and stagnate.
Weakened Immune Function
Your immune system normally keeps the bacteria and fungi that enter your nose from gaining a foothold. When that defense is compromised, sinus infections become more frequent and harder to clear. Conditions that suppress immunity include diabetes (especially when blood sugar is poorly controlled), HIV, cancer and chemotherapy, organ transplant medications, and long-term steroid use. Even temporary immune dips from severe stress, sleep deprivation, or another illness can make you more vulnerable.
Fungal Sinus Infections
Fungi cause a small but important fraction of sinus infections. There are two broad categories. Noninvasive fungal sinusitis is the more common type and stays confined to the nose and sinuses. Allergic fungal sinusitis, a subtype, results from an immune overreaction to fungal spores in the nasal passages. The sinuses fill with thick, sticky mucus, and nasal polyps often form. People with asthma or hay fever are more likely to develop it.
Invasive fungal sinusitis is far less common but far more serious. It can spread beyond the sinuses into the eyes, brain, and surrounding tissue. It occurs almost exclusively in people with severely weakened immune systems: those undergoing chemotherapy, people with unmanaged diabetes, or those taking immunosuppressant medications. Unlike the noninvasive form, invasive fungal sinusitis is life-threatening without treatment.
Other Contributing Factors
Several everyday factors raise your risk without being direct causes. Swimming frequently, especially in chlorinated pools, can irritate the sinus lining. Flying with a cold traps pressure changes in already-congested sinuses. Dry indoor air during winter thickens mucus and slows drainage. Dental infections in the upper teeth can spread to the maxillary sinuses directly above them, a scenario called odontogenic sinusitis that accounts for a meaningful share of one-sided sinus infections.
Gastroesophageal reflux (acid reflux) may also play a role. Stomach acid reaching the back of the throat and nasal passages can irritate the sinus lining, though this connection is still being studied. People with conditions that thicken mucus, such as cystic fibrosis, are particularly prone to chronic sinus infections because their mucus is too viscous to drain properly.