Most sinus infections start with a common cold. A virus inflames the lining of your nasal passages, that inflammation blocks the tiny drainage openings of your sinuses, and mucus gets trapped. From there, bacteria can move in and make things worse. Nearly 29 million adults in the U.S. are diagnosed with sinusitis each year, and while the trigger varies from person to person, the underlying mechanism is almost always the same: something blocks your sinuses from draining properly.
How Sinus Drainage Breaks Down
Your sinuses are air-filled cavities behind your forehead, cheeks, and eyes. They drain through small openings into your nasal passages at a junction called the ostiomeatal complex. The maxillary sinuses behind your cheeks actually have to drain upward, against gravity, relying entirely on tiny hair-like structures called cilia that sweep mucus toward the exit in a coordinated wave.
When something disrupts this system, whether it’s swelling, damage to the cilia, or thickened mucus, secretions pool inside the sinus. That stagnant mucus drops in pH and oxygen levels, creating the warm, low-oxygen environment that bacteria thrive in. The resulting bacterial growth triggers more inflammation, which causes more swelling, which blocks drainage further. This feedback loop is why sinus infections can linger for weeks or keep coming back.
Viruses: The Most Common Starting Point
The vast majority of acute sinus infections are viral. Rhinovirus (the usual culprit behind the common cold), adenovirus, influenza, and parainfluenza are the most frequent offenders. These viruses infect the nasal lining, causing it to swell and produce excess mucus. In most people, the congestion clears on its own within 7 to 10 days.
A viral sinus infection doesn’t need antibiotics and won’t respond to them. The challenge is distinguishing it from a bacterial infection, which is why doctors typically look for symptoms lasting beyond 10 days, or symptoms that initially improve and then suddenly worsen, before considering antibiotics.
When Bacteria Take Over
Bacterial sinus infections are secondary infections. They develop when a virus (or allergies, or another source of inflammation) has already blocked the sinuses and created conditions ripe for bacterial growth. Three species cause the majority of cases: Haemophilus influenzae, Streptococcus pneumoniae, and Moraxella catarrhalis. These bacteria are often already present in your nasal passages in small numbers. They only cause trouble once normal drainage is compromised and they can multiply unchecked.
Bacterial sinusitis tends to produce more intense facial pressure, thicker and more discolored nasal discharge, and sometimes fever. It’s estimated that only about 2% of viral sinus infections progress to a bacterial one, but because so many people get colds each year, bacterial sinusitis is still extremely common.
Allergies and Chronic Inflammation
Allergic rhinitis (hay fever) is one of the biggest ongoing risk factors for recurrent sinus infections. When you inhale an allergen like pollen, dust mites, or pet dander, the blood vessels in your nasal tissue dilate and the lining swells. Your nasal glands ramp up mucus production. Both of these responses narrow or completely block the sinus drainage pathways.
Allergies also directly impair mucociliary clearance, the sweeping action that moves mucus out of the sinuses. Over time, chronic allergic inflammation remodels the nasal tissue, making the swelling more persistent and harder to reverse. This is why people with year-round allergies often cycle through sinus infections repeatedly, especially during peak allergy seasons. Treating the underlying allergy can significantly reduce the frequency of infections.
Structural Problems in the Nose
Anatomy plays a role for some people. A deviated septum, where the wall between your nostrils is significantly off-center, can physically obstruct airflow and sinus drainage on one side. Minor deviations are extremely common and rarely cause issues, but a pronounced deviation can lead to chronic sinusitis on the affected side.
Nasal polyps are another structural factor. These soft, painless growths develop on the lining of the sinuses or nasal passages, often in people with chronic inflammation from allergies or asthma. When polyps grow large enough, they physically block the sinus openings. Other anatomical variations, like unusually narrow drainage passages, can predispose someone to infections even without any other risk factor present.
Fungal Sinus Infections
Fungi cause a small but important subset of sinus infections. The most common type is allergic fungal sinusitis, which occurs when the immune system overreacts to fungal spores breathed in from the environment. The sinuses fill with thick, sticky mucus, and nasal polyps often develop alongside it. People with asthma or hay fever are at higher risk. This form is not dangerous, but it tends to be chronic and difficult to treat.
Invasive fungal sinusitis is far less common and far more serious. In this form, fungi actually destroy tissue inside the nose, and the infection can spread to the eyes and brain. It occurs almost exclusively in people with severely weakened immune systems: those undergoing chemotherapy, living with unmanaged diabetes, taking immunosuppressant medications, or dealing with conditions like HIV. The acute form can progress rapidly and is a medical emergency.
Air Pollution and Tobacco Smoke
Environmental irritants do real, measurable damage to the sinus defense system. Fine particulate matter from air pollution reduces the speed at which cilia beat, and chronic exposure can physically injure or destroy ciliated cells altogether. Without functioning cilia, mucus sits in place, keeping pathogens pressed against the sinus lining for longer periods.
Pollution and cigarette smoke also trigger a structural change in the nasal lining. The body responds by producing more mucus-secreting cells and fewer ciliated cells, a shift that results in thicker, stickier mucus with fewer cilia to clear it. This combination is particularly effective at trapping bacteria and viruses in the sinuses. People who smoke or live in areas with high air pollution have measurably higher rates of both acute and chronic sinusitis.
Other Contributing Factors
Several less obvious factors can set the stage for sinus infections. Swimming, especially in chlorinated pools, can irritate the nasal lining. Flying with a cold can trap pressure in the sinuses and worsen congestion. Dental infections in the upper teeth occasionally spread into the maxillary sinuses directly above them, causing sinusitis that won’t resolve until the tooth is treated.
Immune deficiencies, whether from a medical condition or from medications that suppress the immune system, make sinus infections more frequent and harder to clear. Gastroesophageal reflux has also been linked to chronic sinusitis in some studies, as stomach acid irritating the back of the throat may contribute to ongoing nasal inflammation. And simple dehydration or very dry indoor air can thicken mucus enough to slow drainage, which is why sinus problems tend to spike during winter when heating systems dry out the air.