Sinusitis can develop in response to a virus, bacteria, or fungus that infiltrates the sinuses. The most common causes of a sinus infection are:   • Getting a cold or flu bug • Nasal and seasonal allergies • Nasal polyps (growths) • Having a deviated septum • Having a compromised immune system from medication or illness • Smoking   For babies and young children, using pacifiers or drinking bottles when lying down increases the risk of them developing a sinus infection.