As explained here, among the causes of obstruction or congestion which may lead to sinusitis are the common cold, allergies, certain medical conditions, abnormalities in the nasal passage, and change in atmosphere. Well-Connected Obstruction of the sinus passages may lead to the development of the bacterial infections that cause acute sinusitis, which in certain circumstances may progress into chronic sinusitis. Among the causes of obstruction or congestion are the common cold, allergies, certain medical conditions, abnormalities in the nasal passage, and change in atmosphere. The most common trigger for sinusitis is a viral cold or flu that infects the upper respiratory tract and causes obstruction. Obstruction creates a hospitable environment for bacteria, the primary causes of acute sinusitis.
Fungi are an uncommon cause of sinusitis, but incidence is increasing.
The fungus Aspergillus is the common cause of fungal sinusitis. Others include Curvularia , Bipolaris , Exserohilum , and Mucormycosis. Fungal infections can be very serious and should be suspected in people with sinusitis who also have diabetes, leukemia, AIDS, or other conditions that impair the immune systems. Fungal infections can also occur in patients with healthy immune systems. There have been a few reports of fungal sinusitis caused by Metarrhizium anisopliae, which is used in biological insect control.