What is the cause of sinusitis in general?

Drainage: Sinus health is all about drainage - if they can drain, they'll be fine. When swelling associated with allergies or either a bacterial or viral infection advances enough to interfere with sinus drainage, they can get infected, and pressure can build up into a sinus headache. Once the swelling goes down to the point the sinuses can drain again, they'll heal.

Virus: 98% of upper respiratory infections are due to viruses, for example the rhino virus. The swelling in the lateral wall of the nose then may lead to obstruction of narrow passages that lead into the sinuses in the face and between the eyes. This in general is the pathogenesis of sinusitis. Always starts in the nose and secondarily into the sinuses. Only 2% are complicated by bacteria.