Sinus infections happen when fluid gets trapped in the hollow spaces behind your forehead, cheeks, and eyes, creating an environment where bacteria, viruses, or fungi can thrive. More than 90% of sinus infections start with a virus, typically the same ones responsible for the common cold. Only 2% to 10% of cases involve bacteria.
The Basic Mechanism: Blocked Drainage
Your sinuses are lined with a thin layer of tissue that produces mucus, which normally drains through tiny openings only 1 to 3 millimeters wide. Microscopic hair-like structures on those cells constantly sweep mucus toward the openings and out into your nasal passages. When something causes that lining to swell, whether a cold virus, an allergen, or a chemical irritant, those tiny drainage openings get blocked.
Once drainage stops, mucus pools inside the sinus cavity. The trapped fluid creates negative pressure, oxygen levels inside the sinus drop, and the warm, stagnant environment becomes ideal for bacteria or other organisms to multiply. This is why a sinus infection so often follows a cold: the virus causes the initial swelling, and the resulting blockage sets the stage for a secondary bacterial infection. The blocked drainage, not the germ itself, is the core problem.
Viral Sinus Infections
The vast majority of sinus infections are viral. When you catch a cold, the virus inflames the sinus lining, mucus thickens, and you feel pressure in your face. This type of sinus infection resolves on its own as your immune system clears the virus, usually within 7 to 10 days. Because these infections are viral, antibiotics do nothing for them.
Distinguishing a viral sinus infection from one that has turned bacterial matters because the treatment is different. Current clinical guidelines say a bacterial sinus infection is likely when symptoms like thick nasal discharge, facial pain or pressure, and nasal congestion persist without improvement for at least 10 days, or when symptoms start improving and then suddenly get worse again (sometimes called “double worsening”). If your symptoms follow the typical cold pattern of peaking around days 3 to 5 and then gradually improving, you almost certainly have a viral infection.
How Allergies Set the Stage
Allergies are one of the most common non-viral triggers. When your immune system reacts to pollen, dust mites, pet dander, or mold, it inflames the lining of your nasal passages and sinuses. That swelling narrows or blocks the same tiny drainage openings that viruses target. People with hay fever or asthma have a higher risk of developing sinus infections precisely because their sinuses spend more time in a swollen, poorly draining state.
Chronic or seasonal allergies can create a cycle: repeated inflammation damages the sinus lining over time, making it more vulnerable to infections, which cause further inflammation. Managing the underlying allergy, whether through avoiding triggers or using allergy treatments, reduces the frequency of sinus infections for many people.
Structural Problems That Block Drainage
Some people are prone to sinus infections because of the physical shape of their nasal passages. A deviated septum, where the wall between the two sides of your nose leans to one side, can partially block sinus drainage on the narrower side. Nasal polyps, which are soft, painless growths that form on the lining of the nose or sinuses, can physically obstruct the drainage pathways when they grow large enough. Both conditions lead to repeated sinus infections because mucus has no clear path out.
These structural issues explain why some people get sinus infections several times a year while others rarely do. If you experience frequent infections, particularly always on the same side, an anatomical factor may be contributing.
Fungal Sinus Infections
Fungi, particularly mold and yeast, cause a smaller but distinct category of sinus infections. Tiny fungal spores float in the air constantly, and most people breathe them in without any problem. For people with weakened immune systems, however, these fungi can take hold in the sinuses and cause serious infections. Those at highest risk include people who are HIV positive, undergoing chemotherapy, taking immunosuppressant medications, or living with severe or poorly managed diabetes.
There is also an allergic form of fungal sinusitis that affects people with asthma or hay fever. In this type, the immune system overreacts to fungi in the sinuses, causing chronic inflammation and thick, sticky mucus rather than a traditional infection. This form tends to recur and often requires ongoing management.
Dental Infections That Spread to the Sinuses
Your upper molars and premolars sit directly beneath your maxillary sinuses, the ones in your cheekbones. The roots of these teeth sometimes extend very close to the sinus floor, separated by only a thin layer of bone. When an infection develops in one of these teeth, in the surrounding gum tissue, or as a complication of a dental procedure like a root canal or implant, it can spread directly into the sinus above.
This type, called odontogenic sinusitis, accounts for a meaningful share of chronic or recurring sinus infections. It can be triggered by a tooth abscess, a cyst near an infected tooth, gum disease, or even leftover material from a dental procedure. The key clue is that the infection typically affects only one side and doesn’t respond to the usual sinus treatments. If you have a sinus infection that keeps coming back on the same side, a dental cause is worth investigating.
Other Contributing Factors
Anything that impairs mucus drainage or suppresses your immune system can contribute to sinus infections. Smoking and exposure to secondhand smoke irritate the sinus lining and slow the movement of the tiny hair-like structures that clear mucus. Air pollution and dry indoor air have similar effects. Swimming, particularly in chlorinated pools, can irritate the nasal lining in some people.
Frequent upper respiratory infections in children, often from daycare exposure, make young kids especially prone to sinusitis. Changes in air pressure during flying or scuba diving can also temporarily block sinus drainage and trigger infections in people who are already congested.