Most sinus infections start with a common cold. A virus inflames the lining of your sinuses, the air-filled spaces behind your forehead, cheeks, and eyes. That swelling traps mucus, creating a warm, stagnant environment where bacteria can multiply. Only about 0.5% to 2% of colds in adults progress to a bacterial sinus infection, but when they do, symptoms tend to be more severe and longer-lasting.
How a Sinus Infection Develops
Your sinuses are lined with tiny hair-like structures called cilia that constantly sweep mucus toward your throat, where you swallow it without noticing. This process is your sinuses’ primary defense system: it moves pathogens, allergens, and debris out before they can cause problems.
When something disrupts that drainage, trouble starts. A cold virus, for example, inflames the sinus lining and thickens the mucus. Common respiratory viruses also directly damage cilia and change mucus consistency, making it harder for the remaining cilia to do their job. With mucus pooling instead of draining, bacteria that normally live harmlessly in your nose find ideal conditions to overgrow. The result is pressure, pain, and the thick, discolored discharge most people associate with a sinus infection.
This same basic process, blocked drainage followed by infection, applies regardless of what triggers the blockage in the first place. The cause of the blockage varies, but the chain of events is remarkably consistent.
Viruses vs. Bacteria
Viruses cause the vast majority of sinus infections. The same viruses behind the common cold and flu are responsible, and these infections typically resolve on their own within 7 to 10 days. Bacterial sinus infections are less common but tend to produce more intense symptoms: facial pain concentrated over specific sinuses, thick yellow or green nasal discharge, and symptoms that worsen after an initial improvement or last longer than 10 days.
When researchers have directly sampled fluid from the sinuses of people diagnosed with acute sinusitis, about half show bacterial growth. The three bacteria most frequently identified are the same ones behind many ear infections and cases of pneumonia. But distinguishing a viral sinus infection from a bacterial one based on symptoms alone is difficult, which is why doctors often recommend waiting 10 days before considering antibiotics.
Allergies and Chronic Inflammation
Allergies are one of the most common non-viral triggers. When you breathe in something you’re allergic to, like pollen, dust mites, or pet dander, the tissue inside your nose and sinuses swells. That swelling narrows the drainage passages, and if it persists long enough, trapped mucus becomes a breeding ground for bacteria. People with year-round allergies are especially vulnerable because the inflammation never fully resolves, creating repeated opportunities for infection.
This is why some people get sinus infections seasonally, right alongside their allergy symptoms. Treating the underlying allergy, whether with nasal steroid sprays, antihistamines, or allergen avoidance, can break the cycle by keeping the drainage pathways open.
Structural Problems in the Nose
The physical shape of your nasal passages matters. A deviated septum (where the wall between your nostrils is crooked) can narrow the drainage channels on one side, trapping mucus in the sinuses. Nasal polyps, which are soft, painless growths on the sinus lining, can block drainage the same way.
A deviated septum may cause problems through two routes: it can physically narrow the drainage opening, and it can alter airflow patterns inside the nose in ways that impair the cilia’s ability to move mucus. Either mechanism leads to the same outcome, retained secretions and recurring infections. People with these structural issues often notice their sinus infections happen on the same side of the face or follow a predictable pattern.
Smoking and Air Quality
Smoking increases the risk of chronic sinus problems by more than 40%, even after accounting for other factors like asthma. When smoking and asthma are combined, the risk jumps to 3.6 times that of a non-smoking, non-asthmatic person. Cigarette smoke directly damages cilia, slowing mucus clearance and leaving the sinuses more vulnerable to infection.
Secondhand smoke, heavy air pollution, and regular exposure to chemical fumes or dust can have similar effects. Anything that irritates the nasal lining chronically will impair the drainage system your sinuses depend on.
Why Some Sinus Infections Keep Coming Back
Recurring or chronic sinus infections (lasting 12 weeks or more) often involve a different set of factors than a one-time infection after a cold. One of the most significant is biofilm formation. Bacteria can organize themselves into colonies coated in a protective slime layer that makes them 10 to 1,000 times more resistant to antibiotics than free-floating bacteria.
This resistance works through several mechanisms at once. The slime layer physically blocks antibiotics from reaching bacteria in the deeper layers. Bacteria buried deep within the biofilm slow their metabolism to a near-dormant state, and since most antibiotics work best against actively growing cells, these “persister cells” survive treatment. Even after a course of antibiotics clears the active infection, persister cells can reactivate, migrate to new sites, and seed a fresh biofilm with the same resistant characteristics. This explains the frustrating cycle many people experience: antibiotics seem to work, symptoms improve, then the infection returns weeks later.
Certain bacteria can also hide inside the cells lining the sinuses, adopting a more aggressive form with increased antibiotic resistance. Once internalized, these bacteria act as a reservoir for reinfection, essentially lying in wait for the next opportunity.
Fungal Sinus Infections
Fungi cause a small but important subset of sinus infections. These fall into two broad categories. Non-invasive forms include fungal balls (a clump of fungal material growing in one sinus) and allergic fungal sinusitis, where the immune system overreacts to fungal spores in the sinuses. These types don’t invade surrounding tissue and are generally manageable.
Invasive fungal sinusitis is far more dangerous. It spreads beyond the sinuses into bone, blood vessels, and nearby structures like the eyes and brain. It occurs almost exclusively in people with weakened immune systems: those undergoing chemotherapy, organ transplant recipients on immunosuppressive drugs, people with uncontrolled diabetes (particularly during a crisis called ketoacidosis), and those with blood cancers. The most common culprits are fungi in the Aspergillus and Mucor families. Older adults may also be at increased risk simply due to age-related changes in immune function.
Acid Reflux and Other Systemic Conditions
Gastroesophageal reflux disease (GERD) is an underrecognized contributor to chronic sinus problems. When stomach acid travels far enough up the esophagus, it can reach the back of the throat and nasal passages, causing irritation and inflammation that mimics or worsens sinusitis. This is considered an “atypical” presentation of reflux, meaning many people with reflux-driven sinus symptoms don’t have obvious heartburn.
The mechanism involves tiny amounts of acidic stomach contents being aspirated into the upper airway, triggering inflammation and creating conditions favorable for infection. This can start a self-reinforcing cycle where inflammation leads to infection, which worsens inflammation, which invites more infection. People with persistent sinus symptoms who don’t respond well to standard treatments may benefit from evaluation for reflux.
Cystic fibrosis is another systemic condition strongly linked to chronic sinusitis. The disease produces abnormally thick, sticky mucus throughout the body, including the sinuses, making normal drainage nearly impossible. GERD is also extremely common in people with cystic fibrosis, with prevalence estimates ranging from 35% to 81%, compounding the sinus problems further.
Immune System and Overall Health
Any condition that suppresses your immune system raises your risk. This includes HIV, diabetes, autoimmune diseases requiring immunosuppressive medications, and even prolonged high-dose steroid use for other conditions. A weakened immune system means your body is slower to fight off the bacteria and fungi that your sinuses normally handle without difficulty.
Dental infections in the upper teeth are another overlooked cause. The roots of your upper molars sit very close to the floor of your maxillary sinuses (the ones in your cheeks). An infection at a tooth root can spread directly into the sinus above it, causing sinusitis that won’t resolve until the dental problem is treated. If you have a sinus infection that affects only one side of your face and doesn’t respond to typical treatment, a dental source is worth investigating.