Most sinus infections start with something simple: a cold, allergies, or anything else that causes the lining of your nasal passages to swell. That swelling blocks the tiny openings that normally let your sinuses drain, trapping mucus inside hollow cavities in your skull. Once mucus sits stagnant long enough, bacteria (or sometimes fungi) begin to multiply in that warm, moist environment, and what started as congestion becomes an infection.
What Happens Inside Your Sinuses
Your sinuses are lined with a thin layer of tissue that constantly produces mucus. Tiny hair-like structures called cilia sweep that mucus toward small drainage openings that connect each sinus to your nasal passages. Under normal conditions, this system keeps your sinuses clean and ventilated.
When something disrupts this process, whether it’s swelling, a physical blockage, or damage to the cilia themselves, mucus stops moving. The trapped mucus drops in pH and oxygen levels, creating exactly the kind of low-oxygen, stagnant environment where bacteria thrive. As bacteria grow, they trigger more inflammation, which causes more swelling, which blocks drainage further. It’s a self-reinforcing cycle that can be difficult to break once it gets going.
The Most Common Starting Point: A Viral Cold
The vast majority of sinus infections begin with a viral upper respiratory infection, a common cold. Cold viruses inflame the nasal lining, which swells enough to partially or fully block the sinus drainage pathways. In most cases, the congestion clears on its own within a week or so and never progresses beyond viral inflammation.
A bacterial sinus infection develops when symptoms persist without improvement for at least 10 days, or when symptoms initially start to get better and then suddenly worsen. That worsening pattern, sometimes called “double sickening,” is a hallmark sign that bacteria have taken hold in mucus that’s been sitting too long. Only a small fraction of colds actually progress to bacterial sinusitis, but because colds are so common, this remains the most frequent path to a sinus infection.
Allergies and Environmental Irritants
Allergies are the second major driver of sinus infections. If you have hay fever or are sensitive to dust mites, pet dander, or mold, the chronic inflammation in your nasal passages keeps those drainage pathways narrowed for weeks or months at a time. This ongoing swelling gives bacteria repeated opportunities to colonize trapped mucus.
Air pollution plays a role too. Research at Johns Hopkins found that mice exposed to fine particulate air pollution (particles 2.5 micrometers or smaller) for 16 weeks showed a breakdown in the protective barrier cells lining their nasal passages. When that barrier weakens, allergens, viruses, and bacteria can penetrate tissue more easily, increasing susceptibility to infection. Cigarette smoke causes similar damage to cilia, slowing or stopping the mucus-clearing mechanism that keeps sinuses healthy.
Structural Problems That Block Drainage
Some people are prone to sinus infections because of the physical shape of their nasal anatomy. A deviated septum, where the wall between the two sides of the nose is significantly off-center, can narrow or block a sinus drainage pathway on one side. Nasal polyps, which are painless soft growths on the sinus lining, can do the same thing. When polyps grow large enough to block the nasal passages, they lead to repeated sinus infections. In rare cases, ongoing infections associated with polyps can cause more serious complications, including infections that spread to nearby bone or even the tissue surrounding the brain.
People with naturally narrow sinus openings may also experience blockages more easily than others, even from mild swelling that wouldn’t cause problems in someone with wider passages.
Dental Infections: A Surprising Cause
One cause most people don’t expect is a dental problem. The roots of your upper back teeth sit very close to the floor of your maxillary sinuses (the large sinuses behind your cheekbones). When one of those teeth develops an infection at the root tip, or when gum disease causes significant bone loss around the tooth, bacteria can spread directly into the sinus above.
This is far more common than most people realize. A large meta-analysis published in the Journal of Endodontics found that roughly half of all maxillary sinus infections may originate from dental sources. Severe bone loss around the upper teeth carried the strongest association, increasing the odds more than 13-fold. If you keep getting sinus infections on one side of your face and antibiotics aren’t fully clearing them, a dental issue is worth investigating.
Fungal Sinus Infections
Not all sinus infections are bacterial. Fungi that are naturally present in the air can sometimes trigger sinus problems, particularly in people with asthma or hay fever. In allergic fungal sinusitis, the immune system overreacts to fungi inside the nose, causing the sinuses to fill with extremely thick, sticky mucus. Nasal polyps frequently develop alongside this condition. This type of infection tends to be chronic and recurrent rather than a one-time event.
Invasive forms of fungal sinusitis are much rarer and primarily affect people with weakened immune systems. For the average person, allergic fungal sinusitis is the more relevant concern.
Why Some Infections Become Chronic
Acute sinus infections typically resolve within a few weeks, with or without antibiotics. But some infections persist for 12 weeks or longer, crossing the threshold into chronic sinusitis. Several factors can drive this.
One increasingly recognized factor is bacterial biofilms. Instead of floating freely in mucus where antibiotics can reach them, bacteria can form dense, structured colonies that attach to the sinus lining and coat themselves in a protective matrix. These biofilms are far more resistant to both antibiotics and your immune system than free-floating bacteria. This is one reason chronic sinusitis can be so stubborn: the infection isn’t fully eliminated even after a course of treatment, and symptoms return once medication stops.
Chronic sinusitis is also more likely when underlying causes like allergies, nasal polyps, or immune deficiencies aren’t addressed. If the drainage pathways keep getting blocked, the cycle of mucus stagnation and bacterial growth simply restarts. People who smoke, live in areas with heavy air pollution, or have conditions like asthma or cystic fibrosis face a higher baseline risk of infections becoming chronic.
Key Risk Factors at a Glance
- Recent cold or upper respiratory infection: the single most common precursor
- Allergies: chronic nasal inflammation narrows drainage pathways
- Smoking or air pollution exposure: damages cilia and weakens the nasal lining
- Nasal polyps or deviated septum: physically blocks sinus drainage
- Dental infections or gum disease: bacteria can spread from upper tooth roots into the sinuses
- Weakened immune system: reduces the body’s ability to clear bacteria or fungi before they multiply
- Asthma: associated with both allergic fungal sinusitis and chronic sinusitis