[New and re-emergent epidemics in the changing surrounding world]

In discussion of the emergence of 'new' diseases or the re-emergence of old ones, much attention has been focused on the appearance of hitherto unknown agents versus the transfer of known agents to new populations. The appearance of a new pathogenic microorganism is often assumed to be the result of a change in its genetic properties. Although this may occasionally be the case, in fact most emergent pathogens have already existed in nature. Some occurring in isolated human populations, others well-established in animal species for centuries. Microbial traffic between animals and man is of special interest. Such cross-species transfer of infectious agents is often the result of changes in human behaviour, or in such human activities as deforestation, dam-building and migration.