Pathogenicity is the ability an organism has to cause disease.  It is composed of two factors, 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.