The moulds are filamentous or mycelial fungi and are composed of branched filaments or hyphae. 1 A hyphal tip showing branching. They reproduce by means of spores produced by asexual cell division or sexual reproduction with different types of spores produced dependent on growth conditions. The yeasts are predominantly oval and unicellular. They reproduce by the asexual process of budding.
Fungi have several disadvantageous roles in human health where they cause disease.
Most are dimorphic but there are also a number of pathogenic yeasts. Dimorphic fungi usually assume the mould form when growing and the yeast form when causing infection. Some fungi are able to establish infection in all exposed individuals, others are opportunists which can ordinarily only cause disease in the compromised host. In some mycoses the form and the severity of the infection depend on the degree of exposure to the fungus, the site and method of entry and the level of immunocompetence of the host. Most infections are caused by environmental moulds but also yeasts that are commensals of man and can cause endogenous infections in the compromised host.