How do we treat Chronic Sinusitis?

Background: Wilson has Sinusitis for a few years and he keeps blowing his nose. After taking some over-the-counter flu cold medicine, his running nose symptom got better for a short period of time. However, the color of his nasal discharge remains yellow for 2 weeks without any signs of improvement. Wilson is concerned if he has Sinusitis.

(1) What are the symptoms of Sinusitis? What causes it?

(2) How can we diagnose and treat Sinusitis? Can it come back?

(3) If one has been suffering from Sinusitis for a long time, would other complications develop? How do we treat Chronic Sinusitis?