Sinus infections almost always start with something blocking the normal drainage of mucus from your sinuses. About 90% of people with a common cold develop some degree of sinus inflammation, making viral upper respiratory infections the single most common trigger. When the narrow drainage passages of your sinuses swell shut, mucus gets trapped, oxygen levels inside the sinus cavities drop, and the warm, stagnant environment becomes a breeding ground for bacteria or, less commonly, fungi.
How a Cold Turns Into a Sinus Infection
Your sinuses are air-filled cavities behind your forehead, cheeks, and eyes, all lined with a thin layer of mucus-producing tissue. Tiny hair-like structures called cilia constantly sweep that mucus toward small openings that drain into the nasal passages. This self-cleaning system works well until something disrupts it.
The most common viruses behind sinus infections are rhinovirus (the usual cold virus), influenza, and coronaviruses. Others include parainfluenza virus, adenovirus, and respiratory syncytial virus. When one of these viruses infects the nasal lining, it triggers inflammation and swelling that can partially or fully block the sinus drainage openings. Mucus backs up, pressure builds, and you get that familiar stuffed, achy feeling across your face.
Most of the time, this viral sinus inflammation clears on its own within 7 to 10 days. If your symptoms persist beyond that window, or if they initially improve and then suddenly worsen, bacteria have likely moved in. The trapped, oxygen-poor mucus is an ideal environment for bacteria that normally live harmlessly in your nose and throat. Once those bacteria multiply inside a sealed-off sinus cavity, the infection shifts from viral to bacterial, and the symptoms tend to intensify: thicker, discolored nasal discharge, more intense facial pain, and sometimes fever.
What Causes the Blockage in the First Place
A viral cold is the most frequent starting point, but anything that swells the sinus lining or physically blocks the drainage pathways can set the stage for infection. The underlying mechanism follows three principles: obstruction of sinus drainage, mucosal swelling, and inflammation. Several common triggers can set this chain in motion.
Allergies. Allergic reactions to pollen, dust mites, pet dander, or mold cause the nasal lining to swell. That swelling can restrict ventilation and obstruct the sinus openings, leading to mucus retention and infection. People with year-round allergies face a chronic version of this problem.
Structural issues. A deviated septum, nasal polyps, or other anatomic variations can narrow or block the drainage pathways permanently. Nasal polyps alone account for a significant share of chronic cases, with allergic polyps being the most common type (roughly 86% of nasal polyps). These structural factors explain why some people get sinus infections repeatedly while others rarely do.
Air pollution and weather. Research from Southern China found that higher concentrations of common air pollutants (nitrogen dioxide, sulfur dioxide, and fine particulate matter) were significantly associated with more chronic sinus disease. Cold, dry air also irritates the nasal lining and can impair the mucus-clearing system.
Cigarette smoke and dehydration. Smoking damages the cilia that sweep mucus out of your sinuses. So does chronic dehydration, which thickens the mucus itself. Both make it harder for your sinuses to drain normally, even without a cold or allergy flare.
Dental infections. The roots of your upper back teeth sit very close to the floor of the maxillary sinuses (the ones behind your cheekbones). A dental abscess or infected tooth can spread directly into the sinus, causing an infection that won’t respond to typical treatment until the dental problem is addressed.
Why Some People Get Recurring Infections
Chronic sinusitis, defined as sinus inflammation lasting 12 weeks or longer, is driven by a longer list of predisposing factors than a one-off acute infection. Common contributors include allergic and nonallergic rhinitis, asthma, nasal polyps, impaired mucus clearance, immune system disorders, and recurrent viral infections. Many people with chronic sinusitis have several of these factors overlapping.
Certain medications can also play a role. Antihistamines dry out the nasal passages, which sounds helpful but can actually thicken mucus and slow ciliary movement, making drainage worse in some people. Frequent use of decongestant nasal sprays (the kind you can buy over the counter) can cause rebound swelling after a few days, trapping mucus all over again.
People who spend time in environments with high airflow, chemical fumes, or dry recycled air (think airplanes, office buildings, or industrial settings) put extra stress on their sinus lining. Over time, repeated exposure to these irritants can damage the cilia enough that even a mild cold overwhelms the drainage system.
Fungal Sinus Infections
Fungi account for a small but important subset of sinus infections. The two main categories are molds and yeasts, with molds being the more common culprit. Aspergillus is the most well-known cause, though more recent studies show that Bipolaris and Curvularia are actually recovered most often in allergic fungal sinusitis.
Who gets fungal sinusitis depends largely on immune status. Invasive fungal sinusitis, the most dangerous form, primarily strikes people with weakened immune systems: uncontrolled diabetes, patients undergoing chemotherapy, those on long-term corticosteroids, or people with HIV/AIDS. Mucormycosis (caused by Mucor and Rhizopus fungi) in a person with uncontrolled diabetes is a classic and particularly aggressive example. These infections can spread beyond the sinuses into surrounding bone and tissue and require urgent treatment.
Not all fungal sinus disease is that severe. A “fungus ball,” a clump of fungal material growing inside a single sinus cavity, typically occurs in otherwise healthy women and almost always involves the maxillary sinus. Allergic fungal sinusitis, another noninvasive form, tends to affect younger people with a history of allergies or asthma. It produces thick, dark, peanut butter-like mucus and often involves nasal polyps on both sides.
The Timeline From Cold to Infection
Understanding the timeline helps you know what you’re dealing with. A typical cold peaks around days 3 to 5, then gradually improves. Sinus pressure and congestion during this window are normal viral inflammation, not a bacterial infection, and antibiotics won’t help.
If symptoms last beyond 7 to 10 days without improvement, or if you experience a “double worsening” pattern (you start feeling better, then suddenly get worse again), bacterial sinusitis becomes the likely explanation. Facial pain that localizes to one side, thick yellow or green discharge from one nostril, fever above 102°F (39°C), and upper tooth pain are all signs the infection has progressed past a simple cold.
Acute bacterial sinusitis typically resolves within 2 to 4 weeks with appropriate treatment. If symptoms drag on past 12 weeks, it crosses into chronic sinusitis territory, which often requires a different approach since the underlying problem is usually persistent inflammation rather than an active infection.