A sinus infection is a condition in which the cavities surrounding the nasal passages become inflamed. The less severe form of the infection can be triggered by a cold or allergies and typically disappears on its own. A sinus infection in itself is not contagious but the virus causing the infection can be contagious. What Causes Sinus Infections 1. Virus: The main cause of sinusitis are viruses. A congested sinus is an ideal breeding ground ...