Virulence is a microbial property that is expressed only in
a susceptible host. Why are some
microbes pathogenic while the majority poses no threat to humans or other
hosts? Are pathogenic and non-pathogenic
microbes different? Why are some
environmental microbes pathogenic despite having no need for their hosts? The fungal kingdom provides insight into potential
answers. Of the more than 1.5 million
fungal species only about 150-300 are pathogenic for humans, and of these, only
10-15 are relatively common pathogens.
In contrast, fungi are major pathogens for plants and insects. These facts pose several fundamental
questions including the mechanisms responsible for the origin of virulence
among the few pathogenic species and the high resistance of mammals to fungal
diseases. This talk will explore the
origin of virulence among environmental fungi with no obvious requirement for
animal association and develop the hypothesis that interactions with non-animal
hosts such as protista selected for traits that, in certain circumstances like
weakened immunity, can allow invasion of mammalian hosts. Furthermore, the presentation will discuss
recent evidence that vertebrate endothermy and homeothermy create a restricted
environment for the overwhelming majority of fungal species and speculate that
pressures from fungal diseases contributed to both the extinctions at the end
of the cretaceous that resulted in the demise of the dinosaurs and to the great
mammalian radiation that followed in the tertiary era. Finally, Dr. Casadevall will comment on the
possibility that climate warming will erode the thermal difference between
mammalian and environmental temperatures, an event that could potentially usher
in new fungal diseases in the late 21st century.
