The language of medical mycology
Dear Friends of the Atlas,
Medical mycology language is our essential tool for communication. Remarkably, several of the most basic principles have not properly been defined, which may lead to misunderstanding. What do we mean exactly with ‘emerging’, or ‘pathogen’? The Nomenclature Working Group of ISHAM (International Society for Human and Animal Mycology) aims to achieve general agreement on names of fungi and diseases and on some general terminology.
One of the terms under discussion is ‘pathogen’. Regarding pathogenicity as a property of the fungus, we distinguish two basic types: the one which delivers a fungal advantage of infecting a vertebrate host (primary pathogen) and the one where the infection does not promote survival of the fungal species, as it has an environmental habitat (opportunistic pathogen). ‘Advantage’ means: having a larger progeny, for example by enhancing distribution so that the fungus can start a new population elsewhere. To this aim, transmission to new hosts is required. In contrast, a single infection by an opportunist without transmission delivers no advantage to the fungus. The two types are associated with different immune responses and infection kinetics (de Hoog et al. 2024a).
Among primary pathogens, two types can be distinguished. Anthropophilic dermatophytes such as Trichophyton rubrum are directly transmitted from host to host. Most human-associated dermatophytes have their origin in domestic animals, and have gone through a host shift to Homo sapiens. Initially such infections on human patients, which are immunologically naïve for this unknown fungus, are inflammatory. During the subsequent epidemic, virulence tends to decrease, similar to what happened with Covid-19. This combination of high-virulent onset followed by epidemic expansion is observed today, for example, in Trichophyton indotineae (Luchsinger et al. 2025; Nenoff et al. 2025; Gupta et al. 2024).
The second type of primary pathogens comprises the dimorphic fungi such as Histoplasma capsulatum. They are referred to as environmental pathogens because they grow happily in the environment, but may use a vertebrate host for efficient distribution within suitable habitats. Infected hosts are not contagious, because after death of the host the fungus returns to the environment. Evolution and adaptation may take place in the fungus’ natural habitat and therefore these fungi tend to become endemic.
The disease of a primary pathogen follows a more or less consistent course, and therefore we name the disease after the fungus. This is not the case with most opportunistic pathogens. The great majority of fungi are not prepared to infect an animal, and are considered opportunistic at most. Infection is coincidental, and the host is usually immunocompromised; the course of disease is different every time. Therefore it is not recommended to name the disease after the fungus.
Not every fungus fits nicely into the bipartition primary / opportunistic. Several opportunists prefer habitats that enable them to infect humans as well. For example, Aspergillus fumigatus grows easily at 37ᵒC, while others have melanin to resist phagocytosis. Aspergillus prefers self-heated degrading plant material, but can also survive in human lungs. Such predisposed fungi occur rather frequently as agents of disease in compromised patients, and infections follow a particular course. We therefore name the infection after the fungus, as aspergillosis, even though the agent is not a primary pathogen. Some fungi such as Candida auris are difficult to classify in either category because we lack understanding of its natural life cycle. Additionally, several fungi are commensals which are present on healthy hosts, becoming invasive only when the host develops a metabolic or immunological disorder. See also the relationship of allergy and infection, reviewed by Zhou et al. (2024).
The proposed system of naming disease has recently been published (de Hoog et al. 2024b). The recommended name of fungus and disease can be found at www.atlasclinicalfungi.org. In the menu bar at the top, click on ‘nomenclature’. This open access database contains several thousands of fungi, covering all currently accepted species with medical relevance. For subscribers of the Atlas, a link is provided to the respective pages in the Atlas, with diagnostic information, references on pathology, and antifungal data.
On behalf on the Atlas team we wish you some
happy festive days and a successful 2025!
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