Many patients with allergies will have problems with recurrent sinus infections, also known as sinusitis.
Sinuses are air-filled cavities inside your skull that normally function to warm and humidify the air that you breathe in. The cavities are lined with the same type of tissue that lines your nasal passages and as such are prone to the same kind of inflammation that can occur with nasal allergies or allergic rhinitis.
If the sinuses become too inflamed and too much mucous builds up inside the sinuses, infection can occur. Sinus infections are commonly caused by bacteria, although molds and fungus can also cause infection especially in patients with allergies to certain types of molds (known as allergic fungal sinusitis). Viruses can also contribute to sinus infections which is why your doctor may not want to prescribe an antibiotic unless you have had symptoms for more than 10 days (the typical duration of a viral infection is about a week).