It is useful to clarify the definitions of pathogenesis and virulence before discussing the topic further.
The term pathogenesis is the ability to cause disease and refers to the entry, primary replication, spread to target organs, and establishment of infection in those target organs. The process by which the microbe replicates in the human host depends on the individual having the necessary cell receptors, the degree of cell and tissue injury, their immune system, and other possible factors. Note that this process occurs in the transmission of all infectious diseases, not just zoonoses.
Virulence is the degree of the pathogenesis of a microbe indicated by case fatality rates, and/or its ability to cause tissue damage in the host. In bacteria, virulence is mediated through a number of factors in the microbial DNA including the ability to adhere and invade the cell or tissue, possession of capsules, endo- or exotoxins, or other factors. The major virulence factor among viruses is the ability of a virus to enter a cell and utilize it for replication of new viruses, as well as using the cell's energy and other functions.
In other words, an organism is considered pathogenic if it is able to cause disease but may exhibit different levels of virulence. All of the zoonoses discussed here are pathogenic, but they do vary in their virulence.
