Sinusitis is when you get inflammation of the sinus lining. It usually results from a viral illness. You may have acute or chronic sinusitis if you experience frequent flare-ups of sinusitis, especially if they happen around the same time each year. Since it frequently develops right after an upper respiratory tract infection, sinusitis is sometimes mistaken for the flu or a cold. Sinusitis is simple to overlook since its symptoms are so like those of the common cold and flu, but if untreated, it may be very uncomfortable.