Fungal Factors Enzymes such as keratinase, the presence of capsule in Cryptococcus neoformans , the ability to grow at 37 C, dimorphism, and other as yet undefined factors contribute to fungal pathogenesis which involves a complex interplay of many fungal and host factors. Since they are widespread in nature and are often cultured from diseased body surfaces, it may be difficult to assess whether a fungus found during disease is a pathogen or a transient environmental contaminant. Before a specific fungus can be confirmed as the cause of a disease, the same fungus must be isolated from serial specimens and fungal elements morphologically consistent with the isolate must be observed in tissues taken from the lesion. In general, fungal infections and the diseases they cause are accidental. Although a great deal of information is available concerning the molecular basis of bacterial pathogenesis, little is known about mechanisms of fungal pathogenesis.
Infection is defined as entry into body tissues followed by multiplication of the organism.
The infection may be clinically inapparent or may result in disease due to a cellular injury from competitive metabolism, elaboration of toxic metabolites, replication of the fungus, or an immune response. The first exposure to fungi that most humans experience occurs during birth, when they encounter the yeast C. albicans while passing through the vaginal canal. During this process the fungus colonizes the buccal cavity and portions of the upper and lower gastrointestinal tract of the newborn, where it maintains a life-long residence as a commensal. Another fungus, Malassezia furfur , is common in areas of skin rich in sebaceous glands. How it colonizes the skin is not known, but both M furfur and C albicans are the only fungi that exist as commensals of humans and are considered part of the indigenous flora.