The most common cause of sinusitis in adults is?

The most common cause of sinusitis in adults is a viral infection. Acute sinusitis often begins after a common cold, when inflammation causes the sinus lining to swell and blocks normal drainage.

Viruses such as rhinovirus, influenza virus, and parainfluenza virus can trigger this inflammation. Fluid then builds up in the normally air-filled sinus spaces, producing congestion, facial pressure, nasal discharge, headache, or a reduced sense of smell. Most viral cases improve without antibiotics.

Bacteria can cause sinusitis, but bacterial infection is much less common than viral illness. It may develop secondarily when swelling and poor drainage allow bacteria already present in the nose and throat to multiply. Symptoms that persist for around 10 days without improvement, or that worsen after initially getting better, can suggest acute bacterial rhinosinusitis.

Another common misunderstanding is that thick or colored mucus automatically proves a bacterial infection. Mucus color alone cannot reliably identify the cause. Sinusitis can also be associated with allergies, structural nasal problems, or dental infections, especially in chronic or one-sided cases.