Is sinusitis contagious?

No: Sinusitis is a bacterial infection. If it was caused by a viral upper respiratory infection (cold) then that is what would be contagious.

Yes: Sinusitis is caused by an infection such as a virus or bacteria and is contagious. Antibiotics treat bacterial infections but would not effect a viral infections. You should not be contagious after 24 hrs of using an antibiotic for a bacterial infection . Your body builds up natural protection to viruses and it may take longer than 24 hrs for you to not be contagious after a viral infection.

Usually not: The bugs causing sinusitis r usually the residents here--they live without causing problem because they exist in small number due the oxygen (toxic to these bugs) keeping them at bay. When nose/sinuses congested by cold/allergies blocking the oxygen exchange which allows the bugs to grow in excess and causes sinusitis. Treat the congestion and sinusitis often resolves w/o antibiotics. Consult doc.