Sinus infections start when something blocks the narrow drainage passages of your sinuses, trapping mucus inside and creating conditions where germs can multiply. More than 90% of sinus infections are caused by viruses, typically the same ones responsible for the common cold. Only about 2% to 10% of cases involve bacteria, though those are the ones most likely to need treatment.
How a Sinus Infection Develops
Your sinuses are air-filled spaces behind your forehead, cheeks, nose, and eyes. They’re lined with a thin layer of tissue that constantly produces mucus, which drains into your nasal cavity through tiny openings. The whole system depends on those openings staying clear and on microscopic hair-like structures called cilia sweeping mucus toward the exits.
When something causes the lining to swell, those drainage openings narrow or close entirely. Mucus gets trapped. As it sits there, the oxygen level inside the sinus drops and the environment becomes more acidic. That stagnant, low-oxygen pocket is ideal for bacteria or other organisms that are already present in small numbers to start growing rapidly. Their growth triggers more inflammation, which causes more swelling, which blocks drainage further. It’s a self-reinforcing cycle.
One anatomical detail makes this worse: the main drainage opening of the maxillary sinuses (the large ones in your cheeks) sits at the very top of each sinus. That means mucus has to drain upward against gravity, relying entirely on the cilia to push it out. Even minor swelling can shut down that uphill exit.
Viral Infections Are the Most Common Trigger
The typical sinus infection begins as a cold. A virus infects the nasal lining, which swells and produces extra mucus. In most people, the immune system clears the virus within 7 to 10 days and the swelling goes down. But in a small percentage of cases, the blocked sinuses become secondarily infected with bacteria. In adults, this viral-to-bacterial progression happens in roughly 0.5% to 2% of cases. In children, the rate is higher, around 5% to 10%.
This is why the 10-day mark matters. If your symptoms haven’t improved at all after 10 days, or if they start to get better and then suddenly worsen, a bacterial infection is more likely. If you’ve been sick for less than 10 days and symptoms are gradually improving, you’re almost certainly dealing with a virus.
Allergies and Chronic Inflammation
Allergies are one of the most common non-viral pathways to a sinus infection. When you inhale something you’re allergic to (pollen, dust mites, pet dander), your immune system launches an inflammatory response in the nasal lining. Immune cells flood the tissue and release chemical signals that cause swelling, increased mucus production, and fluid buildup. The result is the same drainage blockage that a cold virus would cause, just triggered differently.
People with year-round allergies are particularly vulnerable because their nasal lining stays inflamed for extended periods. Over time, this chronic inflammation can change the tissue itself, making it thicker and more prone to swelling with each new exposure. Studies of sinus tissue in people with allergies show a distinctive pattern of immune cell buildup, especially a type of white blood cell associated with allergic reactions, deep in the tissue lining.
Structural Problems That Block Drainage
Some people get repeated sinus infections because of the physical shape of their nasal passages. A deviated septum (where the wall between your nostrils leans to one side) can narrow or block the drainage pathway on that side. Nasal polyps, which are soft, painless growths on the sinus lining, can physically obstruct the openings. Scar tissue from previous sinus surgery can do the same thing.
These structural issues don’t cause infections on their own. They lower the threshold. A mild cold that would drain normally in someone with wide-open passages might turn into a full sinus infection in someone whose drainage is already partially blocked. If you find yourself getting sinus infections several times a year, an anatomical obstruction is one of the first things worth investigating.
Air Pollution and Cigarette Smoke
Breathing in polluted air damages the sinus lining over time. Research from Johns Hopkins Medicine tracked over 6,000 adults and found that long-term exposure to fine particulate matter (tiny airborne particles from vehicle exhaust, industrial emissions, wildfire smoke, and similar sources) significantly increased the risk of chronic sinus problems. People exposed over a five-year period were roughly one and a half times more likely to develop chronic sinusitis. The risk of severe inflammation affecting all four sinus groups was nearly five times higher.
Cigarette smoke has a similar effect. It paralyzes and eventually destroys the cilia that sweep mucus out of the sinuses. Without functioning cilia, mucus sits in place regardless of whether the drainage openings are blocked. Secondhand smoke exposure carries the same risk, which is one reason children in smoking households are more prone to sinus infections.
Why Children Get Sinus Infections Differently
Children’s sinuses aren’t fully developed at birth. The ethmoid sinuses (between the eyes) are the most developed in newborns, but the maxillary sinuses (in the cheeks) don’t begin significant growth until ages 3 to 5. The frontal sinuses (behind the forehead) are the last to develop, starting around age 5 and continuing to grow through adolescence.
This matters because younger children can’t get the same types of sinus infections that adults get. A toddler won’t develop a frontal sinus infection because that sinus barely exists yet. It also means that sinus infections in young children tend to be concentrated in the ethmoid area, which produces different symptom patterns, often swelling around the eyes rather than the forehead pressure adults typically describe. Children also progress from viral to bacterial infections at a higher rate than adults, with 5% to 10% of colds in kids leading to bacterial sinusitis compared to roughly 2% in adults.
Fungal Sinus Infections
A less common form of sinusitis is caused by fungi rather than viruses or bacteria. Fungal spores are everywhere in the environment, and most people breathe them in without any problem. But in some individuals, the immune system overreacts to these spores, triggering intense allergic inflammation inside the sinuses. This condition, known as allergic fungal sinusitis, tends to produce thick, dark, sticky mucus and often involves nasal polyps.
The fungi most commonly involved include Aspergillus species and various environmental molds like Alternaria and Curvularia. Unlike bacterial sinusitis, which often follows a cold, fungal sinusitis tends to develop gradually and become chronic. It’s more common in warm, humid climates where mold exposure is higher, and it affects people with existing allergies or asthma at greater rates.
Other Factors That Raise Your Risk
- Swimming and diving: Water forced into the nasal passages can introduce bacteria and irritate the sinus lining. Chlorinated pool water is a common culprit.
- Dental infections: The roots of your upper back teeth sit very close to the floor of your maxillary sinuses. An infected tooth or dental abscess can spread directly into the sinus above it.
- Dry air: Heated indoor air in winter dries out nasal mucus, making it thicker and harder to drain. This is one reason sinus infections peak during cold months alongside respiratory viruses.
- Immune system problems: People with weakened immune systems, whether from illness or medications, are more susceptible to infections taking hold in the sinuses.
- Flying with congestion: Pressure changes during takeoff and landing can force mucus and bacteria into sinus cavities that would otherwise stay clear.
In most cases, sinus infections result from a combination of triggers rather than a single cause. A person with mild allergies who catches a cold during a high-pollution week is at greater risk than someone dealing with just one of those factors. Understanding which triggers apply to you makes it easier to reduce the frequency of infections over time.