As opposed to bacteria, fungi have a true nucleus and rigid cell walls containing chitin, making them immobile. They are mostly made up of cell formations, within which they can specialise in different ways. Unlike plants, they are unable to supply themselves with energy and live off of substances that have been made by other living organisms. Fungi therefore do not directly belong to the world of plants. They are parasites living off their host's metabolism, which they often damage in due course.

World wide they exist in an unbelievable variety. The small, round forms are called budding fungus or yeast. They exist singly and reproduce by budding. In the case of the long filaments or hyphen one talks of filamentous fungi. These fungi multiply by a continued development of hyphen, by which a branching net called mycelium is created. To be counted among this group are the dermatophytes and the mould fungi. Under certain circumstances, some fungi are able to grow either as yeast or hyphe.

Humans use some of them in the field of biotechnology and the food industry, as, for example, in the production of alcoholic beverages or antibiotics. The metabolism output of some species can, however, also cause harm. Fungi can also cause poisoning and allergies in humans. Certain fungi are a composite part of the natural microflora on skin and mucous membranes.

The fungi causing infections in humans may be divided into three medically relevant groups: dermatophytes, yeast as well as moulds and other fungi. This classification is of practical importance, as not all drugs are effective in dealing with all groups.