Rhinosinusitis is inflammation of the nasal passages and sinuses, and the most common cause is a viral infection. Adults typically get one or two of these infections per year, while children average six to eight. But viruses are only the starting point. Bacterial overgrowth, fungal colonization, allergies, structural quirks in the nose, and environmental irritants can all trigger or sustain sinus inflammation, sometimes for months or years. The global prevalence of chronic rhinosinusitis sits around 8.7%, and that number has been climbing steadily over the past four decades.
Acute vs. Chronic: Why Duration Matters
Rhinosinusitis falls into two broad categories based on how long it lasts. Acute rhinosinusitis refers to inflammation lasting up to four weeks and is overwhelmingly caused by infections, usually viral. Chronic rhinosinusitis (CRS) means symptoms have persisted for 12 weeks or longer and often involves a more complex mix of causes, including immune dysfunction, biofilms, and ongoing environmental exposure. Understanding the timeline helps clarify which causes are at play, because a sinus infection that won’t quit for three months is a fundamentally different problem than a bad cold.
Viral Infections: The Most Common Trigger
The vast majority of acute rhinosinusitis starts with a virus. Rhinovirus is the single most frequent culprit, the same pathogen behind most common colds. Respiratory syncytial virus (RSV) is the second most commonly identified virus in sinus infections, followed by influenza and human coronaviruses. These viruses inflame and swell the sinus lining, block normal mucus drainage, and create a warm, stagnant environment inside the sinuses. Most viral cases resolve on their own within seven to ten days.
When Bacteria Move In
Bacterial rhinosinusitis usually develops as a secondary infection after a virus has already done its damage. The swollen, mucus-filled sinuses become a breeding ground for bacteria that normally live harmlessly in the nose and throat. Three species account for most cases: one that commonly inhabits the upper airway (Haemophilus influenzae), a bacterium often found in the nose and throat of healthy people (Moraxella catarrhalis), and the pneumococcus (Streptococcus pneumoniae).
Doctors suspect a bacterial cause when symptoms last beyond ten days without improvement, or when symptoms initially improve and then suddenly worsen around day five. This “double worsening” pattern is a hallmark of bacterial superinfection. Only about 2% of viral sinus infections progress to a bacterial one, but when they do, the symptoms tend to be more severe: thicker nasal discharge, facial pain concentrated over specific sinuses, and sometimes fever.
Fungal Causes
Fungi can cause rhinosinusitis in two very different ways. The noninvasive forms, which are far more common, include allergic fungal rhinosinusitis and fungal balls (a dense clump of fungal material lodged in a sinus cavity). Allergic fungal rhinosinusitis is essentially an overblown immune reaction to inhaled mold spores. The most frequently identified species include Aspergillus flavus, along with darker-pigmented molds like Alternaria, Curvularia, and Bipolaris.
Invasive fungal sinusitis is rarer and far more dangerous. It occurs primarily in people with weakened immune systems: those with uncontrolled diabetes, patients on chemotherapy, or organ transplant recipients on immunosuppressive drugs. In diabetic patients, fungi in the Mucorales family are the most common invaders. In patients with very low white blood cell counts, Aspergillus species tend to dominate. Invasive forms can cause significant tissue destruction and require urgent treatment.
Allergies and Perennial Triggers
Allergies are one of the strongest risk factors for chronic rhinosinusitis, particularly allergies to perennial (year-round) triggers like dust mites, mold, and pet dander. Among patients with chronic rhinosinusitis and nasal polyps, roughly 56% show sensitivity to at least one perennial allergen, compared to about 5% of the general population. That’s a striking gap. Studies using CT imaging have found sinus inflammation in 68% of people with perennial allergic rhinitis, versus 33% of non-allergic controls.
Seasonal allergies (pollen from trees, grasses, or weeds) show a weaker link to chronic sinus disease. The running theory is that perennial allergens keep the sinus lining in a constant state of low-grade inflammation, which eventually disrupts normal mucus clearance and sets the stage for chronic disease. Seasonal allergens, by contrast, only flare for a few weeks or months and may not sustain enough damage to tip the balance.
Structural Problems in the Nose
The sinuses drain through narrow openings into the nasal cavity, and anything that narrows those pathways can trap mucus and promote infection. A deviated septum, where the wall between the two nasal passages is significantly off-center, is one of the most common structural contributors. Other variations include enlarged turbinates (the bony ridges inside the nose that warm and humidify air), an air-filled expansion of the middle turbinate called a concha bullosa, and extra or unusually shaped sinus cells near the drainage pathways.
These anatomical features don’t guarantee you’ll develop rhinosinusitis. Plenty of people have a deviated septum and never have sinus problems. But when combined with another trigger, like a viral infection or allergies, a narrow drainage pathway can turn a mild episode into a prolonged one. This is why surgical correction of structural issues is sometimes recommended for people with recurrent or chronic sinus disease that hasn’t responded to medication.
Environmental and Occupational Irritants
The sinus lining is the body’s first contact point with everything you inhale, and chronic exposure to airborne irritants can damage that lining enough to trigger persistent inflammation. Cigarette smoke, including secondhand smoke, is one of the best-documented risk factors. It impairs the tiny hair-like structures (cilia) that sweep mucus out of the sinuses, disrupts the immune cells embedded in the sinus lining, and weakens the barrier function of the tissue itself.
Occupational exposures carry similar risks. Workers regularly inhaling dust, fumes, formaldehyde, solvents, volatile organic compounds, or heavy metals like chromium and nickel face higher rates of chronic sinus disease. Air pollution more broadly, including fine particulate matter, has been linked to both the development and worsening of rhinosinusitis. Even indoor exposures like woodstove smoke and animal dander in farming environments contribute. These irritants can push an already inflamed sinus lining past the tipping point from acute to chronic disease.
How Chronic Inflammation Sustains Itself
In chronic rhinosinusitis, the initial trigger often fades into the background while the immune system’s response becomes the primary problem. The inflammation takes on a life of its own, driven by different immune pathways depending on the person. The most studied pattern, called type 2 inflammation, involves a cascade where the sinus lining releases signaling molecules that activate specific immune cells, particularly eosinophils, mast cells, and certain T cells. These cells pump out proteins that promote mucus overproduction, tissue swelling, and, in many cases, the formation of nasal polyps.
Nasal polyps are soft, painless growths that hang from the sinus lining and can block airflow and drainage. Type 2 inflammation drives polyp formation by causing fluid and protein to accumulate in the tissue, creating the grape-like masses that characterize chronic rhinosinusitis with nasal polyps. This form of the disease is strongly associated with asthma and aspirin sensitivity, forming a triad that suggests a shared underlying immune dysfunction.
Not all chronic rhinosinusitis follows the type 2 pattern. Some patients show type 1 inflammation, which is linked to viral infection responses and involves different immune cell populations. Others show type 3 inflammation, associated with bacterial infections and driven by neutrophils rather than eosinophils. These distinctions matter because they respond to different treatments, and they help explain why chronic rhinosinusitis can look so different from one patient to the next.
Biofilms: Why Some Infections Won’t Clear
One reason chronic rhinosinusitis resists treatment is bacterial biofilms. Instead of floating freely in mucus where antibiotics can reach them, bacteria in chronic sinus infections often organize into structured colonies coated in a protective matrix. This biofilm acts like armor. It shields the bacteria from the immune system and from antibiotics, which can’t penetrate the matrix effectively. Bacteria deep inside a biofilm also enter a dormant state, making them invisible to drugs that target actively dividing cells.
Biofilms can also pass antibiotic resistance genes between bacterial species within the colony, accelerating the development of resistant infections. This is a major reason why some people cycle through multiple courses of antibiotics without lasting improvement. The biofilm persists on the sinus lining, and once the antibiotic course ends, the bacteria reactivate and symptoms return. Addressing biofilms often requires a combination of approaches, sometimes including surgery to physically remove the infected tissue and restore sinus drainage.
Other Contributing Factors
Several additional conditions raise the risk of rhinosinusitis. Immune deficiency, whether inherited or caused by medications, makes it harder for the body to clear infections from the sinuses. Gastroesophageal reflux has been identified as a predisposing factor, likely because acid reaching the back of the throat and nasal passages can irritate and inflame the sinus lining. Asthma frequently coexists with chronic rhinosinusitis, sharing overlapping inflammatory pathways in the upper and lower airways. Conditions affecting mucus consistency, like cystic fibrosis, dramatically increase the risk of chronic sinus disease because thick, sticky mucus can’t drain properly through the sinus openings.