Most sinus infections start with something you’ve already had: a common cold. A virus inflames the lining of your nasal passages, that swelling blocks the tiny drainage openings of your sinuses, and trapped mucus becomes a breeding ground for bacteria or fungi. Roughly 90% to 98% of all sinus infections are viral, meaning only a small fraction, about 2% to 10%, involve bacteria at all.
How a Cold Turns Into a Sinus Infection
Your sinuses are air-filled cavities behind your forehead, cheekbones, and the bridge of your nose. Each one drains through a narrow opening into your nasal passages. The maxillary sinuses under your cheekbones have it especially tough: their drainage openings sit at the top of the cavity, so mucus has to travel upward against gravity. Tiny hair-like structures called cilia beat in coordinated waves to push mucus out. When this system works, your sinuses stay clear.
When a cold virus infects the nasal lining, inflammation swells the tissue around those narrow drainage openings. Once an opening is blocked, mucus pools inside the sinus with nowhere to go. That warm, moist, stagnant environment is ideal for bacteria. About 2% of adults and 10% of children with a cold or upper respiratory infection go on to develop a secondary bacterial sinus infection. The bacteria most commonly involved are the same ones that cause ear infections and pneumonia in everyday life.
Allergies and Chronic Inflammation
Allergies are one of the most common non-viral paths to a sinus infection. When you inhale something you’re allergic to, like pollen, dust mites, or pet dander, your immune system overreacts. Mast cells in your nasal lining release histamine and other inflammatory chemicals within minutes. Blood vessels dilate, fluid leaks into surrounding tissue, and the spongy structures inside your nose swell dramatically. This is the stuffiness you feel during allergy season.
That swelling narrows or completely closes the same drainage openings a cold would block. But allergies also trigger a second wave of inflammation four to eight hours later, which recruits more immune cells and prolongs the congestion. If you have chronic allergic rhinitis (persistent nasal allergies), your sinuses may be partially blocked for weeks or months at a time, creating repeated opportunities for infection.
Structural Problems That Block Drainage
Some people get recurrent sinus infections because of the physical shape of their nasal anatomy. A deviated septum, where the wall between your nostrils is off-center, can narrow one side enough to impede airflow and drainage. Nasal polyps, soft growths that develop on the lining of your sinuses or nasal passages, can do the same thing. When polyps grow large enough, they physically obstruct the sinus openings and lead to repeated infections.
These structural issues don’t cause infection on their own, but they make every cold, allergy flare, or minor irritation more likely to escalate. If you find yourself getting sinus infections several times a year, a structural problem is one of the things worth investigating.
How Smoking Damages Your Sinuses
Cigarette smoke attacks the sinus drainage system at a fundamental level. The cilia that sweep mucus out of your sinuses are physically damaged by smoke exposure. Their beat frequency slows, and their length shrinks by at least 10%. That last 10% of each cilium’s length is the part that actually contacts and pushes the mucus layer forward, so even a small reduction in length significantly impairs clearance.
Smoke also causes oxidative stress that changes the mucus itself, making it thicker and harder to move. The combination of slower, shorter cilia trying to push thicker mucus means your sinuses clear themselves far less efficiently. Mucus sits longer, and bacteria have more time to multiply. This is why smokers and people regularly exposed to secondhand smoke get sinus infections more often.
Fungal Sinus Infections
Fungi are everywhere in the air, and most people breathe them in without any problem. But under certain conditions, fungi can colonize the sinuses and cause infection. People with asthma or hay fever are more prone to allergic fungal sinusitis, where the immune system overreacts to fungal spores trapped in the sinuses.
More serious fungal sinus infections tend to affect people whose immune systems are compromised. This includes people who are HIV positive, undergoing chemotherapy, living with unmanaged diabetes, or taking immunosuppressant medications including long-term steroids. In these cases, fungi can invade the sinus tissue itself, which is a medical emergency that requires aggressive treatment.
Swimming, Diving, and Pressure Changes
Water activities create a less obvious path to sinus infection. When you dive underwater, every foot of descent adds roughly half a pound of pressure per square inch to your tissues. If you can’t equalize the pressure in your sinuses (by clearing your ears and sinuses during descent), the resulting vacuum engorges and ruptures tiny blood vessels in the sinus lining. Blood pools inside the sinus cavity and acts as a growth medium for bacteria.
The reverse can happen on the way up. If mild swelling from a cold or allergies has narrowed the sinus openings, expanding air can get trapped on ascent, stretching the sinus lining painfully and causing further tissue damage. This is why diving with even mild congestion is a significant risk factor for sinus barotrauma and subsequent infection. Frequent swimmers also face chlorine irritation of the nasal lining, which can contribute to chronic inflammation over time.
Acute Versus Chronic Sinusitis
The distinction matters because the causes and management differ. Acute sinusitis lasts less than four weeks and is almost always triggered by a viral infection. It typically resolves on its own. Chronic sinusitis persists beyond 12 weeks, sometimes with symptoms that wax and wane but never fully clear. Chronic cases are more likely to involve a combination of factors: underlying allergies, nasal polyps, immune deficiencies, or repeated bacterial colonization that never fully resolves.
People who fall between these two categories, with symptoms lasting four to twelve weeks, are sometimes described as having subacute sinusitis. This middle ground often represents an acute infection that’s been slow to resolve, possibly because the underlying drainage problem (structural, allergic, or environmental) hasn’t been addressed.
Who Gets Sinus Infections Most Often
The risk factors overlap in ways that explain why some people seem to get sinus infections constantly while others rarely do. Having allergies, nasal polyps, and regular smoke exposure together creates a situation where sinus drainage is compromised on multiple fronts simultaneously. Add a common cold to that baseline, and infection becomes nearly inevitable.
Other less obvious risk factors include dental infections (upper teeth sit directly below the maxillary sinuses, and infection can spread upward through the thin bone between them), immune deficiencies that may be subtle enough to go undiagnosed, and spending time in dry indoor air that thickens nasal mucus. People with asthma also have higher rates of sinusitis, likely because the same inflammatory patterns that affect the lower airways affect the sinuses too.