is the ability an organism has to cause disease. It is composed of
infectivity, or the ability to
infect and colonize a host and virulence,
or the ability to cause host cell damage. Infectivity is measured
as infectious dose, meaning the number of
microbes necessary to initiate infection, colonization and disease.
Highly pathogenic forms such as the Ebola virus can infect a host if only
one virus comes in contact with skin or mucus membranes, while others such
as the spores of Bacillus anthracis, require 8000-10,000 spores
to cause a serious illness. Virulence can occur along a spectrum
(left) where highly pathogenic organisms always cause disease while less
virulent forms may not cause disease at all or act as opportunistic pathogens.