Fungi, although everywhere, are not often considered human pathogens. That is, they rarely infect humans with a normal immune system. The patients who then have problems with fungi either have an over or underactive immune system.
Patients whose immune systems are compromised can often suffer life-threatening fungal infections of the sinuses. These patients are extremely sick and require emergent surgery and IV antifungal medications.
Patients who have an overactive immune system seem to be the chronic sinus sufferers. Classic allergy to molds seems to be a major cause of chronic rhinosinusitis. See Diagnosing Mold and Fungal Allergies and Treating Mold and Fungal Allergies for information.
More recently researchers from the Mayo Clinic have discovered an exaggerated cell mediated reaction to the fungus Alternaria. Unfortunately, Alternaria seems to be ubiquitous, and it has a very short germination cycle so it presents new antigens in the nose relatively quickly. Patients respond abnormally to the fungi by calling out eosinophils into the nasal secretions. Eosinophils are a type of white blood cell usually involved in allergic reactions or parasitic infections. In this case the eosinophils attack the Alternaria and release a product called MBP (Major Basic Protein). The MBP kills the fungi but it is also very toxic to the nasal mucosa. It can cause anywhere from mild problems to overt nasal polyposis. Even though there are lots of eosinophils, these patients may or may not have allergies.