Many microorganisms that affect humans have originated in animal species and have subsequently evolved to afflict humans (1). Many factors may contribute to the ability of infectious agents to cross the species barrier and include those related to the host, the pathogen and the host-pathogen environment. The microorganism must be able to establish itself in the new host within an environment that allows transmission to take place. Transmission may be facilitated by proximity, degree of physical contact between animals and humans, and the ability to be inoculated into the new host. Several examples of such pathogens have been recently encountered and include the agents of avian influenza (H5N1 and H9N2), severe acute respiratory syndrome, hantavirus infection and variant Creutzfeldt-Jakob disease. A recent comprehensive literature review (2) identified 1415 species of infectious microorganisms, including 217 viruses and prions, 538 bacteria and rickettsia, 307 fungi, 66 protozoa and 287 helminths, known to infect humans. Of these, almost two of three are zoonotic (ie, capable of being transmitted between humans and animals). Of the 1415 species of infectious microorganisms, 175 are considered to be ‘emerging’ pathogens (2). Of these, 132 (75%) are zoonotic, with protozoa and viruses considered more likely to be ‘emerging’ infectious agents than other taxa. Among infectious agents associated with the ‘emerging’ infectious diseases, RNA viruses are most likely to emerge as the greatest threat. Given the similarity of genetic, physiological and behavioural characteristics of humans and nonhuman primates, the latter are considered high-risk sources of emerging infectious agents with the capacity to infect humans. Primate-to-human cross-species transmission of infectious agents has become a focus of signifi