A sinus infection almost always starts with swelling that blocks the tiny drainage openings of your sinuses. Once those openings close off, mucus gets trapped, oxygen levels drop inside the sinus cavity, and bacteria or other pathogens that are normally flushed out begin to multiply. The whole process can unfold over a matter of days, and it usually begins with something as ordinary as a cold.
Your Sinuses Depend on Drainage
You have four pairs of sinuses: maxillary (behind your cheeks), frontal (behind your forehead), ethmoid (between your eyes near the bridge of your nose), and sphenoid (deep behind your nose). Each one is lined with a thin layer of mucus-producing tissue and connects to your nasal cavity through a small opening called an ostium. These openings are narrow to begin with, some only a few millimeters wide.
Under normal conditions, tiny hair-like structures called cilia beat in a coordinated wave, pushing mucus through these openings and out into the nose. This mucus acts as a conveyor belt, trapping dust, allergens, and bacteria, then sweeping them away before they can cause problems. It’s the first line of defense against airborne pathogens. When both the mucus and the cilia are working properly, your sinuses stay clear and ventilated.
The Blockage That Sets Everything Off
The most common trigger is a viral upper respiratory infection, a regular cold. When a cold virus inflames the nasal lining, the tissue around those sinus openings swells. This is especially concentrated in an area called the ostiomeatal complex, where the maxillary, frontal, and anterior ethmoid sinuses all drain through a shared corridor. Even a small amount of swelling in this zone can seal off multiple sinuses at once.
Once the opening is blocked, three things happen simultaneously. Mucus keeps being produced but has nowhere to go, so it pools inside the sinus. The cilia, now bathed in thickened, stagnant mucus, can’t beat effectively. And oxygen inside the sealed cavity drops, creating a low-oxygen, slightly acidic environment where bacteria thrive. What was a minor viral irritation becomes a breeding ground.
Colds aren’t the only thing that can trigger this cascade. Allergies cause the same kind of mucosal swelling. A deviated septum or nasal polyps can physically narrow the drainage pathways, making blockage more likely even from mild inflammation. Environmental irritants like cigarette smoke, air pollution, and dry indoor air also damage the mucosal lining and slow ciliary movement. Dental infections in the upper teeth can spread to the maxillary sinus directly, since the roots sit close to the sinus floor.
When a Cold Becomes a Sinus Infection
The vast majority of sinus infections start as viral, and most resolve on their own. Only about 0.5% to 2% of all upper respiratory infections progress to a bacterial sinus infection. Among people who do develop acute sinusitis, roughly one-third have a bacterial cause rather than a purely viral one.
The timeline matters. A typical cold peaks around days three to five and gradually improves. If your symptoms persist for 10 days or more without getting better, that’s the pattern most associated with bacterial infection. There’s also a pattern called “double sickening,” where you start to improve after the initial cold, then get noticeably worse again around days five to six with a return of fever, worsening headache, or increased nasal discharge. A third pattern involves severe onset: a high fever of 102°F or above along with thick, discolored nasal discharge and facial pain that lasts at least three consecutive days from the start. Any of these three patterns suggests bacteria have taken hold in the trapped mucus.
What Happens Inside the Blocked Sinus
Bacteria that normally live harmlessly in small numbers on the nasal lining now have ideal conditions to multiply. The stagnant mucus provides nutrients, and the lack of airflow means the immune system’s usual clearance mechanisms are compromised. White blood cells flood the area, producing the thick yellow or green discharge that’s characteristic of infection. This inflammatory response actually makes things worse in the short term: the swelling increases, the mucus gets thicker, and ciliary function degrades further.
Inflammation driven by the immune response itself damages the cilia and alters the consistency of mucus, making it stickier and harder to move. This creates a feedback loop where the body’s attempt to fight the infection perpetuates the conditions that allow it to persist.
Why Some Infections Keep Coming Back
In some cases, bacteria form biofilms on the sinus lining. A biofilm is a colony of bacteria encased in a protective matrix of proteins, sugars, and DNA. This shell shields the bacteria from both the immune system and antibiotics. Bacteria in biofilms can survive exposure to antibiotic concentrations up to a thousand times higher than what would kill the same bacteria floating freely. The interior of a biofilm is low in oxygen and acidic, conditions that further reduce antibiotic effectiveness.
Biofilms are a major reason chronic sinusitis (lasting 12 weeks or more) is so difficult to treat. Oral antibiotics often can’t penetrate the sinus tissue well enough to reach the biofilm, and even when they do, the protected bacteria survive. People with biofilm-related sinusitis tend to have more severe inflammation and poorer outcomes from standard treatments compared to those without biofilms.
People at Higher Risk
Anything that impairs mucus clearance raises your risk. Allergies are the most common predisposing factor because they cause chronic low-grade swelling in the nasal passages. Structural issues like a deviated septum, nasal polyps, or scarring from prior sinus surgery physically narrow the drainage pathways. Smokers damage both their cilia and their mucus composition, slowing clearance significantly.
People with immune deficiencies, even subtle ones, are more susceptible to recurrent infections. Conditions that fundamentally alter mucus or ciliary function, like cystic fibrosis or primary ciliary dyskinesia, lead to severe and repeated sinus infections because the basic clearance mechanism is broken from the start.
How Sinuses Develop in Children
Children don’t have the same sinus anatomy as adults. The ethmoid and maxillary sinuses are present at birth and continue growing through childhood, which is why young children can still get sinus infections. The frontal sinuses don’t develop until around age seven, and the sphenoid sinuses don’t fully form until adolescence. This means the pattern and location of sinus infections in children shifts as they grow, with younger kids experiencing infections primarily in the ethmoid and maxillary sinuses. Children are also more prone to sinus infections in general because their immune systems are still maturing and they encounter frequent viral illnesses in school and daycare settings.