Most sinus infections start with a common cold. A virus inflames the lining of your sinuses, blocks their drainage pathways, and traps mucus inside, creating the pressure and pain you feel behind your cheeks, eyes, or forehead. But viruses aren’t the only trigger. Bacteria, fungi, allergies, and even the physical shape of your nasal passages can all set the stage for a sinus infection.
Viruses Start the Majority of Cases
The overwhelming majority of acute sinus infections are viral. The same pathogens that cause the common cold, particularly rhinoviruses, are the most frequent culprits. Influenza, parainfluenza, coronaviruses, adenoviruses, and respiratory syncytial virus can also trigger sinus inflammation. Rhinovirus alone accounts for more than 100 known strains and can survive on surfaces for up to four days, which is why sinus infections often follow a handshake, a doorknob, or a sick family member.
When one of these viruses reaches the tissue lining your sinuses, it triggers swelling and a flood of mucus. Normally, tiny hair-like structures called cilia sweep mucus out of the sinuses through small drainage openings. Swelling narrows or blocks those openings, mucus pools, and the stagnant environment becomes a breeding ground for further infection.
When Bacteria Move In
A bacterial sinus infection is almost always a secondary event, meaning bacteria take advantage of the damage a virus already caused. The three bacteria most commonly involved are Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. These organisms are often already present in your nasal passages in small numbers, but once viral inflammation blocks sinus drainage, they can multiply rapidly in the trapped mucus.
The timing of your symptoms is the most reliable way to tell the difference between a viral and bacterial infection. If your symptoms have lasted fewer than 10 days and are gradually improving, a virus is likely responsible. A bacterial infection becomes more likely when symptoms persist beyond 10 days without any improvement, or when you start getting better and then suddenly worsen again. This “double worsening” pattern, where a cold seems to improve around day five or six before returning with heavier congestion and facial pain, is a hallmark of bacterial sinusitis.
How Allergies Set the Stage
Allergies don’t directly cause sinus infections, but they create the conditions that make one far more likely. When you’re exposed to an allergen like pollen, dust mites, or pet dander, your nasal lining releases a cascade of inflammatory chemicals. These cause the tissue inside your nose and sinuses to swell, ramp up mucus production, and impair the cilia that normally keep things draining. The result is the same bottleneck that viruses create: blocked sinuses filled with stagnant mucus, ripe for bacterial growth.
This is why people with seasonal or year-round allergic rhinitis tend to get more sinus infections than those without allergies. The inflammation is essentially chronic, meaning the sinuses are partially blocked even before a cold or other trigger pushes them over the edge. Managing allergy symptoms with antihistamines, nasal steroid sprays, or allergen avoidance can reduce the frequency of sinus infections for people caught in this cycle.
Structural Problems in the Nose
The physical architecture of your nasal passages matters. A deviated septum, where the wall dividing your nasal cavity leans to one side, can narrow the pathways through which sinuses drain. This creates a mechanical bottleneck, but it also changes how air flows through the sinuses, which can impair the cilia’s ability to move mucus out efficiently. Either way, the result is retained secretions and a higher risk of infection.
Nasal polyps, which are soft, painless growths on the sinus lining, cause similar problems. They physically obstruct drainage openings and trap mucus behind them. People with both polyps and a deviated septum face a compounding effect, where multiple blockages make it extremely difficult for sinuses to clear themselves naturally. In some cases, surgical correction of these structural issues is the only way to break the cycle of recurrent infections.
Fungal Sinus Infections
Fungi are a less common but important cause of sinusitis, particularly in people with weakened immune systems. Molds are responsible more often than yeasts, with species like Aspergillus, Bipolaris, and Curvularia being the most frequently identified. A separate group of fungi, Mucor and Rhizopus, cause a rare but life-threatening condition called mucormycosis that is most common in people with uncontrolled diabetes.
Diabetes, chemotherapy, long-term steroid use, and immune suppression in general all increase the risk of fungal sinusitis. However, not all fungal sinus infections require a compromised immune system. Fungus balls, which are dense clumps of fungal material that grow inside a single sinus cavity, most commonly occur in otherwise healthy women and typically affect the maxillary sinuses behind the cheeks. Another form, granulomatous invasive fungal sinusitis, also tends to appear in people with normal immune function.
Smoking and Air Pollution
Tobacco smoke is particularly damaging to the sinuses because it directly attacks the clearance system that keeps them healthy. The cilia lining your sinuses beat in coordinated waves to push mucus toward the drainage openings. Cigarette smoke impairs both the growth of new cilia and the chemical signals that control their beating. At low levels of exposure, cilia may initially beat faster as a compensatory response, but at higher or prolonged exposure, the damage becomes pathological and clearance breaks down.
Secondhand smoke is nearly as harmful. A study of children with chronic sinusitis who underwent sinus surgery found a striking difference in recovery based on smoke exposure. Children who were not exposed to secondhand smoke saw their ciliary coverage increase significantly after surgery, from about 27% to 35% of the sinus lining. Children exposed to secondhand smoke showed almost no improvement, going from roughly 10% to 10.5%. The smoke essentially prevented their sinuses from healing.
Why Some Sinus Infections Won’t Go Away
Chronic sinusitis, defined as sinus inflammation lasting 12 weeks or longer, affects millions of people. It accounts for about 2.7 million doctor visits per year in the United States alone. One major reason chronic infections resist treatment is the formation of bacterial biofilms. These are organized colonies of bacteria that encase themselves in a protective slime layer on the sinus lining, fundamentally different from the free-floating bacteria that antibiotics are designed to kill.
Bacteria inside biofilms are 10 to 1,000 times more resistant to antibiotics than bacteria floating freely. The protective matrix acts as a physical barrier, preventing antibiotics from reaching the bacteria embedded deep inside. The slime layer can even contain enzymes that break down antibiotics before they penetrate. Deeper inside the biofilm, bacteria enter a dormant, slow-growing state, which makes them even harder to kill since most antibiotics work best against actively dividing cells. These dormant “persister cells” can survive high antibiotic concentrations, then reactivate and seed new infections elsewhere in the sinuses, which is why some people experience recurring infections that feel identical each time.
Genetic Conditions That Affect the Sinuses
Some people are predisposed to chronic sinus infections because of inherited conditions that affect mucus or cilia. Cystic fibrosis causes the body to produce abnormally thick, sticky mucus due to a defect in a gene that controls chloride transport across cell membranes. This dense mucus overwhelms the cilia and clogs the sinuses, creating a persistent environment for bacterial colonization.
Primary ciliary dyskinesia is a separate genetic condition where the cilia themselves are structurally defective and unable to beat properly. It affects roughly 1 in 15,000 people. About half of those with primary ciliary dyskinesia also have a condition called Kartagener syndrome, which involves chronic sinusitis, lung damage, and a mirror-image reversal of internal organs. Both conditions impair the same fundamental mechanism, the mucociliary clearance system, and result in recurrent sinus and lung infections starting in childhood.