Immunity and the emergence of virulent pathogens.

The emergence/re-emergence of infectious diseases has been one of the major concerns for human and wildlife health. In spite of the medical and veterinary progresses as to prevent and cure infectious diseases, during the last decades we have witnessed the emergence/re-emergence of virulent pathogens that pose a threat to humans and wildlife. Many factors that might drive the emergence of these novel pathogens have been identified and several reviews have been published on this topic in the last years. Among the most cited and recognized drivers of pathogen emergence are climate change, habitat destruction, increased contact with reservoirs, etc. These factors mostly refer to environmental determinants of emergence. However, the immune system of the host is probably the most important environmental trait parasites have to cope with. Here, we wish to discuss how immune-mediated selection might affect the emergence/re-emergence of infectious diseases and drive the evolution of disease severity. Vaccination, natural (age-associated) and acquired immunodeficiencies, organ transplantation, environmental contamination with chemicals that disrupt immune functions form populations of hosts that might exert specific immune-mediated selection on a range of pathogens, shaping their virulence and evolution, and favoring their spread to other populations of hosts.