Infectious outbreaks in human populations frequently occurred during the last decades. Most of these epidemics were emerging or re-emerging diseases and about 75% of them were caused by animal pathogens mainly from wild animal species. Thus the "species barrier" dogma seems to be frequently in the wrong. Many factors are probably involved in this failure. Among them, genetic mutations mainly in viruses which are more and more frequently discovered. On the other hand, anthropic factors are the major causes of this species barrier breaches. Closer contacts with domestic and/or wild animals, dramatic growth of human populations, poverty, numerous immunodepressed people, international travels and trading are facilitating or determining factors of the inter-specific transmission of pathogens. International mobilization is mandatory to efficiently anticipate and control the emergence of such zoonoses. According to their unpredictable emergence and their rapid potential worldwide spreading, their control requires an international cooperation both in a permanent epidemiological surveillance and a rapid efficient riposte. Human and veterinary medical institutions have to closely cooperate in this issue.