[How to define the species barrier to pathogen transmission?]

A given microbial pathogen usually targets a restricted number of animal species. Some pathogens can be transmitted to humans from another animal species, either directly (rabies, brucellosis, etc.) or through a vector (Lyme's disease, West Nile fever, etc.). Few infectious agents with animal reservoirs infect humans, and even fewer are capable of human-human transmission. This is attributed to the "species barrier", a simplistic concept that in fact involves a series of conditions for successful inter-species transmission. These include access to an infectable surface, multiplication on that surface, colonisation, invasion, multiplication inside the new host, and resistance to innate and adaptive immune mechanisms. Each of these steps requires a specific ligand-receptor interaction. The full series of events must be "reprogrammed" for efficient implantation in a new host. These changes occur through mutations or genetic exchanges. Direct human-to-human transmission often requires additional adaptive modifications.