Sinus infections (also known as sinusitis or rhinosinusitis) are caused when germs begin to grow in sinuses that are blocked and filled with fluid. This blockage of the nasal cavities occurs by inflammation of the sinuses and is commonly caused by a virus. In other cases, a sinus infection is caused by bacterial or fungus. Although there are different medications for treating sinus infections, no specific vaccine protects you against the infection. AFC Urgent Care ...