Sinus infections develop when the small openings that drain your sinuses get blocked, allowing mucus to pool and germs to multiply in the warm, moist environment. The most common trigger is a simple cold: the virus inflames the sinus lining, swells the drainage passages shut, and trapped mucus becomes a breeding ground for bacteria or other organisms. But colds are just one of several routes to a sinus infection.
What Happens Inside Your Sinuses
Your sinuses are air-filled pockets behind your forehead, cheeks, nose bridge, and deep behind your nose. They’re lined with a thin layer of tissue that constantly produces mucus, which tiny hair-like structures called cilia sweep toward small openings that drain into your nasal passages. When everything works, mucus flows out steadily and bacteria never get a foothold.
A sinus infection starts when that drainage system breaks down. Anything that swells the lining or physically blocks the openings, from a cold virus to allergies to a structural issue, traps mucus inside. Bacteria that normally pass through harmlessly now have a warm, stagnant pool to colonize. As they multiply, your immune system responds with more inflammation, which further blocks drainage and creates a cycle that can be hard to break without intervention.
Viruses Are the Most Common Cause
Most sinus infections are viral, meaning they’re caused by the same viruses responsible for the common cold. The virus itself inflames the sinus lining, and for many people the congestion and facial pressure resolve on their own within 10 days. This is technically viral sinusitis, and antibiotics won’t help.
The trouble comes when a viral infection lingers or worsens. If your symptoms haven’t improved at all after 10 days, or if you start getting better and then suddenly get worse again, the infection has likely become bacterial. Two bacteria account for roughly 70% of these cases: one commonly found in the respiratory tract and another that frequently colonizes the nose and throat. Bacterial sinus infections are less common than viral ones but tend to cause more intense pain and thicker, discolored discharge.
Allergies and Chronic Inflammation
Allergic rhinitis (hay fever) is one of the strongest ongoing risk factors for sinus infections. When your immune system reacts to pollen, dust mites, pet dander, or mold, the resulting inflammation swells the sinus lining in much the same way a cold virus does. The difference is that allergies can keep that swelling going for weeks or months, giving bacteria repeated opportunities to take hold. Researchers describe the nose, sinuses, and lungs as a “unified airway,” meaning chronic allergic inflammation in one area tends to affect the others.
People with asthma or year-round allergies are especially prone to repeated sinus infections because their baseline level of inflammation is already elevated, leaving less room for normal mucus drainage.
Structural Blockages
Sometimes the problem is mechanical. A deviated septum, where the wall between your nostrils leans significantly to one side, can narrow or completely block a sinus drainage pathway. Nasal polyps, which are soft, painless growths on the sinus lining, can do the same thing. Bone spurs inside the nose are another culprit.
These structural issues don’t cause infections on their own, but they reduce the margin for error. A minor cold that would clear easily in someone with wide-open drainage passages can turn into a full sinus infection when the passages are already partially obstructed. People with a severely deviated septum often deal with chronic sinusitis that keeps coming back until the structural problem is corrected.
Dental Infections
This one surprises many people: a significant number of sinus infections start in the teeth. The roots of your upper back teeth sit very close to the floor of your maxillary sinuses (the ones behind your cheeks). An infected tooth, a root canal gone wrong, or even a dental implant procedure can introduce bacteria directly into the sinus.
Studies indicate that more than 40% of maxillary sinus infections have a dental origin. When the infection affects only one side, that number climbs above 70%. A sinus infection that keeps coming back on one side, particularly with a foul taste or smell, is worth investigating with a dentist as well as an ENT doctor.
Air Pollution and Cigarette Smoke
Breathing polluted air damages the sinus lining over time in ways that make infection more likely. Research from Johns Hopkins found that mice exposed to airborne particulate matter developed significantly more inflammation in their sinuses, including a thicker lining and weakened connections between the cells that form the sinus barrier. When those cellular connections break down, bacteria, viruses, and allergens can penetrate more easily.
Cigarette smoke, whether firsthand or secondhand, has a similar effect. It paralyzes the cilia that sweep mucus out of the sinuses, slows drainage, and keeps the lining in a state of chronic irritation. Smokers and people regularly exposed to secondhand smoke get more sinus infections and take longer to recover from them.
Fungal Sinus Infections
Fungi are a less common but important cause, particularly for people with weakened immune systems. In most cases, fungal sinusitis is noninvasive, meaning the fungus stays confined to the sinuses. This includes allergic fungal sinusitis, where the immune system overreacts to fungal spores in the air, and fungal balls, where fungi clump together and physically block a sinus. People with asthma or hay fever are more likely to develop the allergic type.
Invasive fungal sinusitis is rare but serious. It can destroy tissue inside the nose and spread to the eyes and brain. This form occurs primarily in people whose immune systems are compromised by cancer treatment, uncontrolled diabetes, or immunosuppressive medications. It requires urgent medical treatment.
Why Children Get Sinus Infections Differently
Children’s sinuses develop on a staggered timeline, which affects which infections they can get and when. The sinuses near the nose bridge and behind the cheeks are present at birth and keep growing through childhood. The forehead sinuses don’t develop until around age 7, and the sinuses deep behind the nose don’t form until adolescence. Younger children can only get infections in the sinuses that have already developed, and the symptoms often look different from adult sinusitis, more closely resembling a cold that simply won’t quit.
Kids also get more colds than adults (six to eight per year on average), and each cold is another opportunity for the drainage system to fail. Daycare attendance, exposure to secondhand smoke, and enlarged adenoids all raise the risk further.
The Pattern That Leads to Chronic Sinusitis
A single acute sinus infection lasts less than four weeks. When infections keep recurring or symptoms persist for 12 weeks or longer, the condition is considered chronic sinusitis. This usually involves a combination of the factors above: ongoing allergies, structural problems, or repeated infections that never fully resolve between episodes. The sinus lining becomes permanently thickened and inflamed, the cilia stop working efficiently, and the sinuses lose their ability to clear mucus on their own. Breaking this cycle often requires addressing the underlying cause, whether that’s managing allergies, removing polyps, or correcting a deviated septum, rather than just treating each infection as it arrives.