A good definition of human pathogen is ‘a microbial or parasitic species that can infect and is capable of causing disease in humans under natural transmission conditions’ (Woolhouse, 2006, p. 511).
The best-known bacterial pathogens have co-evolved with their hosts to subvert host functions and they possess special virulence factors that have indispensable functions in
mediating host–pathogen interactions.
Aspergillus infections do not resemble classical bacterial diseases. A. fumigatus has no obvious need for passage in humans or other animals. The fact that Aspergillus can become a deadly pathogen may be a biological accident associated with it extreme opportunism.
With invasive aspergillosis, the immunological status of the host – not the virulence of the fungus – is the single most significant element. Despite intensive study, the molecular basis of the pathogenic potential of A. fumigatus remains elusive (Brakhage and Langfelder, 2002)
Source: An Overview of the Genus Aspergillus Joan W. Bennett