Sinus infections develop when the hollow spaces behind your forehead, cheeks, and eyes become blocked and filled with fluid, creating conditions where viruses or bacteria thrive. More than 90% of sinus infections start with a virus, usually the same ones responsible for the common cold or flu. Only 2% to 10% of cases involve bacteria.
How Blocked Sinuses Become Infected
Your sinuses are normally lined with a thin layer of mucus that traps dust, germs, and other particles. Tiny hair-like structures called cilia sweep that mucus toward your nose and throat, keeping the sinuses clean and ventilated. When something disrupts this drainage system, whether it’s swelling from a cold, allergies, or a physical obstruction, mucus pools inside the sinus cavities. The glands lining those cavities keep producing mucus regardless, and the backed-up fluid becomes an ideal breeding ground for bacteria. Bacteria multiply rapidly in the stagnant mucus, your immune system responds with inflammation, and the resulting swelling blocks drainage even further. It’s a self-reinforcing cycle.
This is why most sinus infections follow a cold. The virus inflames and swells the sinus lining, drainage slows or stops, and the trapped mucus becomes a petri dish. A cold that seems to improve and then gets worse around day 7 to 10, or one that lasts beyond 10 days without improvement, has likely progressed to a bacterial sinus infection.
Colds and Respiratory Viruses
The most common route to a sinus infection is catching a respiratory virus. Cold and flu viruses spread through droplets when an infected person coughs, sneezes, or talks, and through touching contaminated surfaces and then touching your nose or eyes. Once the virus takes hold in your nasal passages, the inflammation it causes can extend into the sinuses. In most people, this viral sinus congestion clears on its own within 7 to 10 days. In a smaller percentage, the prolonged blockage allows bacteria already present in your nose to colonize the sinuses, turning a viral cold into a bacterial sinus infection.
Sinus infections themselves aren’t directly contagious, but the viruses and bacteria that trigger them are. You can pass a cold virus to someone who never develops a sinus infection, or you can catch a virus from someone who doesn’t have one either. Whether that virus progresses to a sinus infection depends on your anatomy, immune system, and other individual factors.
Allergies and Chronic Swelling
Allergies are one of the most reliable pathways to repeated sinus infections. When you inhale an allergen like pollen, dust mites, or mold, your immune system launches an inflammatory response in the nasal lining. That swelling narrows or closes off the sinus drainage openings just as effectively as a cold virus does. People with year-round allergies live with low-grade swelling that keeps their sinuses perpetually vulnerable.
Over time, chronic allergic inflammation can damage the protective lining of the sinuses. The barrier between sinus tissue and the outside environment becomes more permeable, allowing allergens and microbes easier access. This dysfunction can also promote the growth of nasal polyps, which are soft, noncancerous growths that physically block the drainage pathways. Once polyps form, they tend to trap mucus and create a persistent cycle of infection and inflammation that’s difficult to break without medical treatment.
Structural Problems in the Nose
Some people are more prone to sinus infections because of the physical shape of their nasal passages. A deviated septum, where the wall between your nostrils is significantly off-center, can narrow or block sinus drainage on one side. This is extremely common. Most people have some degree of septal deviation, but a severe one can lead to chronic sinusitis by preventing mucus from draining normally.
Nasal polyps, narrow sinus openings, and bone spurs inside the nose can all have the same effect. If you get sinus infections repeatedly on the same side, a structural issue is worth investigating. These problems don’t cause infections directly, but they create the conditions where infections are more likely by keeping mucus trapped.
Dental Infections
This is one that catches people off guard. The roots of your upper back teeth sit very close to the floor of your maxillary sinuses, the large sinuses behind your cheekbones. An infected tooth, an abscess, or even certain dental procedures can introduce bacteria directly into the sinus cavity. Studies indicate that more than 40% of maxillary sinus infections originate from dental problems. When the infection is on just one side, that figure rises above 70%.
A sinus infection that affects only one side of your face, especially if it’s accompanied by a foul smell or taste, may have a dental origin rather than a respiratory one. These infections won’t resolve with typical sinus treatments until the underlying tooth problem is addressed.
Diving and Swimming
Water activities can trigger sinus infections through a different mechanism. When you dive underwater, every foot of descent adds roughly half a pound of pressure per square inch on your tissues. Your sinuses are air-filled cavities, and if you can’t equalize the pressure (by clearing your ears and sinuses), the increasing water pressure creates a vacuum effect inside the sinus. This causes the tiny blood vessels lining the sinuses to swell and burst, filling the cavity with blood. That pooled blood then serves as a growth medium for bacteria, leading to infection.
Swimmers who aren’t diving deep can still develop sinus infections from contaminated water being forced into the nasal passages, particularly in lakes, rivers, or poorly maintained pools.
Weakened Immune Defenses
Your immune system normally keeps the bacteria in your nasal passages in check. When that system is compromised, those bacteria can more easily gain a foothold in the sinuses. People with antibody deficiencies, particularly low levels of IgA and IgG (the immune proteins that patrol mucous membranes), develop chronic sinusitis at roughly twice the rate of people with normal immune function: 12% compared to 5% in one large study.
Conditions that suppress the immune system, including diabetes, HIV, chemotherapy, and long-term steroid use, all increase susceptibility. Smoking also counts here. It paralyzes the cilia that sweep mucus out of the sinuses and damages the mucosal lining, combining immune suppression with impaired drainage.
Fungal Sinus Infections
A small subset of sinus infections are caused by fungi rather than viruses or bacteria. Allergic fungal sinusitis occurs in people with normal immune systems who develop a localized allergic reaction to fungal spores growing in areas of poor sinus drainage. The immune response produces thick, dense mucus packed with inflammatory cells that’s difficult for the body to clear. This type almost always occurs alongside nasal polyps and tends to be chronic and recurring.
Fungal sinus infections are far less common than viral or bacterial ones, but they’re worth knowing about if you have nasal polyps, known fungal allergies, and sinus symptoms that don’t respond to standard treatments.
Acute Versus Chronic Sinusitis
How you get a sinus infection shapes what kind you end up with. A single episode triggered by a cold is acute sinusitis, defined as symptoms lasting up to four weeks. The hallmarks are colored nasal drainage, facial pressure or pain, and congestion. Most acute infections resolve on their own or with minimal treatment.
When symptoms persist for 12 weeks or longer, it’s classified as chronic sinusitis. Chronic cases are more often driven by allergies, structural problems, immune deficiencies, or fungal involvement rather than a single viral infection. The distinction matters because the treatment approach is fundamentally different. Acute sinusitis is usually about managing a temporary blockage, while chronic sinusitis requires identifying and addressing the underlying cause of persistent drainage failure.