In basic terms – sinusitis is an infection of one or many of the paranasal sinuses. There are many potential causes of sinusitis, including viral, bacterial and fungi. This is extremely important because antibiotics frequently are not needed to treat viral and fungal causes, but are often essential in treating bacterial sinusitis. The most common presentation of bacterial sinusitis one that starts with a viral upper respiratory infection that begins to improve, unfortunately the patient then develops some or all of the following symptoms: purulent nasal discharge, frontal facial pain, upper tooth pain, and sinus tenderness. Decreased or abnormal sense of smell or taste is also very common. Allergic disease is a common predisposing factor for sinusitis, as are nasal polyps and structural abnormalities of the sinuses themselves. Usually the diagnosis can be made by a thorough history and physical, however recurrent or chronic infections may need a CAT scan of the sinuses, rhinoscopy, or both. There are both acute and chronic forms of sinusitis, they are described further below.