The fungi are admirably equipped with an array of degradative and lytic weaponry to colonize and enter the animal body. The host, of course, has an equally impressive armoury not only of defensive, but also aggressive machinery to prevent such an invasion. Very few fungi are obligate animal pathogens, but many are opportunists, able to take advantage of any chink in the host's defences.

Fungal infections of humans

The dermatophytes

The dermatophytes are a group of fungi which are particularly well adapted for the infection of the superficial keratinized structures of the body, including the nails, hair shafts and the stratum corneum (the cornified layer) of the skin. The true dermatophytes all belong to one of three genera of fungi, and are all able to use keratin as a source of nutrients. They are distinguished from a number of other fungi which may colonize the same superficial sites, including Candida albicans, Malassezia furfur and Aspergillus spp., by the fact that the dermatophytes are not opportunists. They are well adapted to their pathological niche and able to infect the immunocompetent individual.

The three genera of dermatophytes are Trichophyton, Epidermophyton and Microsporum, which together contain over 40 species. These organisms are relatively similar, but can be distinguished by colony morphology, macroscopic appearance and some biochemical tests. Recently, discovery of the perfect, or sexual, stage of several species of dermatophyte has led to their reclassification into two new genera, Arthroderma, containing the perfect Trichophyton species, and Nannizzia, containing the perfect Microsporon species.