A sinus infection happens when the small drainage openings in your sinuses get blocked, trapping mucus inside and creating conditions where bacteria, viruses, or fungi can thrive. Over 90% of sinus infections start with a common virus like a cold, and only 2% to 10% of cases involve bacteria.
How Your Sinuses Normally Work
You have four pairs of hollow, air-filled spaces behind your face: the frontal sinuses (behind your forehead), the maxillary sinuses (behind your cheekbones), the ethmoid sinuses (between your eyes), and the sphenoid sinuses (deeper behind your nose). These cavities are lined with a thin layer of tissue that constantly produces mucus. That mucus serves a purpose: it traps dust, allergens, and germs, then drains out through narrow passageways into your nasal cavity. From there, you swallow it without thinking about it.
The critical piece of this system is a set of tiny drainage channels called the ostiomeatal complex. This pathway handles outflow from three of the four sinus pairs. Millions of microscopic hair-like structures called cilia line the inside of the sinuses, beating in coordinated waves to push mucus toward these drainage openings. When this system works smoothly, germs get flushed out before they can cause trouble.
What Goes Wrong: The Blockage-to-Infection Cycle
A sinus infection begins when something disrupts that drainage system. The trigger is usually swelling. When the tissue lining the nasal passages becomes inflamed, whether from a cold, allergies, or irritants like cigarette smoke, the narrow drainage openings swell shut. Those openings are only a few millimeters wide to begin with, so even a small amount of swelling can seal them off.
Once the drainage path closes, mucus starts pooling inside the sinus cavity. At the same time, the cilia may stop working properly, either because of the infection itself or because the mucus has become too thick to move. The stagnant mucus drops in oxygen levels and becomes more acidic. This creates an ideal breeding ground for bacteria that were already present in small numbers. Those bacteria multiply rapidly in the warm, sealed environment, triggering more inflammation, which causes more swelling, which blocks drainage even further. It’s a self-reinforcing cycle.
Viruses Are the Most Common Trigger
The vast majority of sinus infections are caused by the same viruses responsible for the common cold. When a cold virus infects your nasal lining, it causes inflammation and excess mucus production. In most people, this resolves on its own within 7 to 10 days. But in some cases, the viral inflammation blocks the sinuses long enough for bacteria to take hold as a secondary infection.
This is why bacterial sinus infections almost always follow a cold rather than appearing on their own. The telltale pattern is a cold that seems to be improving, then suddenly worsens after about a week, with new facial pain, thicker or discolored mucus, and sometimes fever. That “double worsening” often signals that bacteria have moved in. The two most common bacterial culprits account for roughly 70% of bacterial cases combined.
Allergies, Anatomy, and Other Risk Factors
Anything that causes chronic swelling in the nasal passages raises your risk of sinus infections. Allergies are one of the most common culprits. Seasonal or year-round allergic reactions keep the nasal lining inflamed for weeks or months, repeatedly narrowing those drainage channels. People with untreated nasal allergies get sinus infections more frequently for exactly this reason.
Structural issues in the nose also play a role. A deviated septum, where the wall between the two sides of your nasal cavity is significantly off-center, can partially block sinus drainage on one side and lead to chronic sinusitis. Nasal polyps, which are soft, noncancerous growths that develop on the lining of the sinuses, can physically obstruct the drainage openings. In some people with nasal polyps, fungi naturally present in the air trigger an allergic-type immune response that keeps the sinuses chronically inflamed.
Other factors that increase risk include upper respiratory infections, swimming or diving (which can force water and bacteria into the sinuses), dental infections in the upper teeth (which sit just below the maxillary sinuses), and any condition that weakens the immune system.
Acute vs. Chronic Sinus Infections
Not all sinus infections follow the same timeline, and the duration matters because it often reflects different underlying causes.
- Acute sinusitis lasts up to four weeks. This is the most common type and usually follows a cold. Most cases resolve without antibiotics.
- Subacute sinusitis lasts four to twelve weeks. It represents a gray zone where the infection or inflammation hasn’t fully cleared but hasn’t become truly chronic.
- Chronic sinusitis lasts twelve weeks or longer. At this point, the problem is less about a single infection and more about persistent inflammation. The causes tend to shift from simple viral or bacterial triggers toward structural issues, allergies, immune dysfunction, or fungal involvement.
Some people also experience recurrent acute sinusitis, defined as four or more separate episodes per year with symptom-free periods in between. This pattern typically points to an underlying risk factor like allergies or a structural problem that keeps setting the stage for new infections.
Why Most Cases Don’t Need Antibiotics
Because 90% to 98% of sinus infections are viral, antibiotics won’t help the overwhelming majority of cases. Viruses don’t respond to antibiotics at all. Even when bacteria are involved, mild bacterial sinus infections often resolve on their own as drainage is restored.
The main treatments that actually help are ones that restore drainage. Saline nasal rinses physically flush out trapped mucus. Nasal steroid sprays reduce the swelling that caused the blockage in the first place. Staying hydrated helps keep mucus thin enough for the cilia to move. Decongestant sprays can temporarily shrink swollen tissue, though using them for more than a few days can cause rebound swelling that makes the problem worse.
Bacterial sinus infections are more likely when symptoms last longer than 10 days without improvement, when symptoms are severe from the start (high fever, intense facial pain, very discolored nasal discharge), or when symptoms improve and then sharply worsen. These are the situations where antibiotics may genuinely help, because the body’s normal clearing mechanisms have failed and bacteria have established themselves in the trapped mucus.