Most sinus infections start with a common cold. Viruses are the leading cause, but bacteria, fungi, structural problems in the nose, dental infections, and a weakened immune system can all trigger or prolong a sinus infection. Understanding what’s behind yours matters because the cause shapes whether it clears on its own or needs treatment.
A sinus infection, or sinusitis, develops when fluid builds up in the air-filled pockets behind your forehead, cheeks, and eyes. That trapped fluid becomes a breeding ground for germs, and the sinus lining swells in response. The result is the familiar combination of facial pressure, congestion, and thick nasal discharge.
Viral Infections: The Most Common Cause
Viruses cause the majority of sinus infections. The same respiratory viruses responsible for the common cold, including rhinoviruses, influenza, and parainfluenza, inflame and swell the nasal passages. That swelling narrows the small openings that normally let your sinuses drain, trapping mucus inside. In most cases, a viral sinus infection resolves on its own within 7 to 10 days as the cold runs its course.
The challenge is that a viral sinus infection feels almost identical to an early bacterial one. Both produce congestion, facial pain, and discolored mucus. Clinicians use a few patterns to tell the difference: symptoms lasting 10 or more days without improvement, a high fever (102°F or above) with intense facial pain and thick discharge in the first three days, or a “double sickening” pattern where you start improving around day five or six, then suddenly get worse again. If none of those patterns apply, the infection is almost certainly viral and antibiotics won’t help.
When Bacteria Take Over
Bacterial sinus infections typically don’t start from scratch. They develop as a secondary infection after a virus has already caused swelling and mucus buildup. With drainage blocked and oxygen levels low inside the sinuses, bacteria that normally live harmlessly in the nose find ideal conditions to multiply.
Three species account for the vast majority of acute bacterial cases. One type of streptococcus is responsible for roughly 30 to 40 percent of cases, a type of haemophilus for 20 to 30 percent, and moraxella for 12 to 20 percent. These are common respiratory bacteria, not exotic pathogens, which is why most bacterial sinus infections respond well to standard antibiotics when they’re truly needed.
One reason some bacterial sinus infections become chronic is biofilm formation. Bacteria can organize themselves into a protective, three-dimensional structure that coats the sinus lining. This biofilm acts as a physical shield against antibiotics and the body’s own immune defenses. Bacteria periodically break free from the biofilm and trigger new rounds of inflammation, which is why some people experience repeating cycles of infection that never fully resolve.
Structural Problems That Block Drainage
Anything that physically narrows or blocks the drainage pathways of your sinuses raises your risk of infection. The two most common structural causes are nasal polyps and a deviated septum.
Nasal polyps are soft, painless growths on the lining of the nasal passages or sinuses. Small polyps may cause no symptoms at all, but when they grow large enough to block sinus drainage, they create the same trapped-mucus environment that viruses do. The result is repeated sinus infections that keep coming back even after treatment. In rare cases, ongoing infections linked to polyps can lead to serious complications, including infection spreading to the bone around the sinuses or to the tissue surrounding the brain.
A deviated septum, where the wall between your nostrils is significantly off-center, can block one side of the nose more than the other. This makes the sinuses on that side especially vulnerable to poor drainage and recurrent infection. Other structural causes include narrowed sinus openings from previous surgery or injury, and in children, enlarged adenoids.
Fungal Sinus Infections
Fungi are everywhere in the air we breathe, and for most people they’re harmless. But in certain situations, fungi can colonize the sinuses and cause infection. There are two broad categories: noninvasive and invasive.
Noninvasive fungal sinusitis is the more common type and stays confined to the nose and sinuses. It takes several forms. Allergic fungal sinusitis happens when your immune system overreacts to fungi in the nasal passages, filling the sinuses with thick, sticky mucus and sometimes triggering polyp growth. A fungal ball, or mycetoma, occurs when fungi clump together inside a sinus cavity and gradually grow large enough to block drainage. A third type involves fungi growing on dried mucus crusts inside the nose without invading tissue.
Invasive fungal sinusitis is far less common but far more dangerous. In its acute form, fungi destroy blood vessels inside the nose, killing tissue and potentially spreading rapidly to the eyes and brain. This type is a medical emergency. Chronic and granulomatous forms progress more slowly but still damage nasal tissue over time. People at highest risk for invasive fungal sinusitis include those who are HIV positive, undergoing chemotherapy, living with poorly controlled diabetes, or taking immunosuppressive medications including long-term steroids.
Dental Problems and Sinus Infections
Your upper back teeth sit remarkably close to the floor of your maxillary sinuses, the large sinus cavities behind your cheekbones. Sometimes only a thin layer of bone separates tooth roots from the sinus lining. This proximity means that an infected tooth, a deep cavity, or even certain dental procedures can introduce bacteria directly into a sinus.
Dental-related sinus infections account for 10 to 40 percent of all chronic maxillary sinusitis cases. When only one side of the face is affected, dental causes are even more likely, responsible for up to 75 percent of unilateral maxillary sinus infections. These infections are often missed because the tooth pain may be subtle or absent, and the sinus symptoms look the same as any other sinusitis. The key clue is one-sided symptoms, especially if standard sinus treatments aren’t working. Treatment requires addressing the dental source, not just the sinus infection itself.
Environmental and Lifestyle Triggers
Your sinuses rely on a self-cleaning system: tiny hair-like structures called cilia beat in coordinated waves to push mucus (and the germs trapped in it) out of the sinuses and toward the throat. Anything that slows or damages this system leaves mucus sitting in place longer, giving bacteria more time to multiply.
Cigarette smoke is one of the most potent disruptors of this process. Smoke exposure slows the beating of cilia and reduces the number of cilia that are actively functioning. When combined with heavy alcohol use, the effect is compounded. Research shows that co-exposure to cigarette smoke and alcohol decreases both the speed and the total number of beating cilia within six hours. This helps explain why smokers and heavy drinkers experience more frequent and more severe sinus infections.
Air pollution, strong chemical fumes, and very dry air can also irritate the sinus lining and impair mucus clearance, though their effects are generally less dramatic than direct smoke exposure.
Allergies and Chronic Inflammation
Allergies are one of the strongest risk factors for recurring sinus infections. Respiratory allergies are present in 31 to 72 percent of people with chronic or recurrent sinusitis. The connection is straightforward: allergic reactions swell the nasal lining, narrow the drainage pathways, and increase mucus production, creating the same stagnant conditions that invite infection.
Seasonal allergies, dust mite sensitivity, pet dander reactions, and mold allergies can all keep the nasal passages in a near-constant state of inflammation. Over time, this chronic swelling can lead to tissue changes that make the sinuses structurally more prone to blockage, even between allergy flare-ups. Managing the underlying allergy often reduces sinus infection frequency more effectively than treating each infection individually.
Immune System Deficiencies
If you get sinus infections frequently and they’re difficult to clear, an underlying immune deficiency may be a factor. Your body produces several types of antibodies that help fight infections in the respiratory tract. When levels of these antibodies are low, the sinuses lose a critical layer of defense against bacteria and other pathogens.
Pooled antibody deficiencies have been identified in about 13 percent of people with recurrent chronic sinusitis and 23 percent of those with sinusitis that doesn’t respond well to treatment. Deficiencies in specific antibody subtypes are even more common in this population, found in 5 to 50 percent of patients depending on the type measured. Common variable immunodeficiency, a condition where the body produces inadequate amounts of protective antibodies, is one of the more frequently identified disorders in people with stubborn, recurring sinus infections.
Interestingly, some treatments for other conditions can contribute to the problem. Long-term use of systemic corticosteroids, often prescribed for autoimmune diseases or severe asthma, can suppress antibody production and increase the breakdown of existing antibodies. This creates a cycle where the medication used to control inflammation simultaneously weakens the immune defenses that prevent sinus infections.