What is the cause of a sinus infection?

Bacterial vs. Viral: Typically bacterial or viral in origin. Viruses tend to resolve on their own in a few days (think of the common cold) whereas bacterial infections persist longer than a week and/or there's a "double-worsening" where one starts to feel better, and then the symptoms relapse. Antibiotics like Azithromycin and Augmentin (amoxicillin and clavulanate) are used to treat bacterial sinus infections.

Cause of Sinus: Anything that irritates the nasal and sinus membranes for longer than a few days is what causes sinus infections. A common cold, smoke, acid reflux, all can lead to inflammation and the resultant swelling. Swelling leads to blockage of sinus passages.