The evolution of viruses, the emergence of viral diseases: a synthesis that Martinus Beijerinck might enjoy.

The relentless production of viral variants and their selection for improved "fit" are seen from the perspective of the infectious disease sciences as ever-changing viral phenotypes and emerging disease risks. In the Darwinian cause:effect equation, we can characterize very well the effects of mutation and selection--these are catalogued as new viral phenotypes or pathotypes. However, the selective forces themselves driving such changes remain rather mysterious. Many selective forces must be at work, acting on the virus, the host, the host population and the environment. In some instances the virus seems to test new unoccupied niches in the absence of any apparent environmental change, but usually it is clear that changes are driven by human activity. Most important must be the ever increasing density of human, domestic animal and crop plant populations and the consequent increased opportunities for transmission of viral variants. Also important must be the great changes affecting all ecosystems--these especially favor the emergence of new zoonotic viruses and viral "species jumpers." The great increase in human travel and transport carries exotic viruses, vectors and hosts around the world, again favoring viral occupation of new niches. The rise of bioterrorism adds yet another threat. Increasing numbers of emerging viral disease episodes seem to be linked to a decline in global resources for proven public health programs, agricultural extension programs, and the like, programs that have stood in the way of the spread and evolution of viral pathogens. If the relationship between viral evolution and the emergence of new viral diseases is rooted firstly in the host and the host population, then more research and resources must be directed to intervention at these levels rather than at the level of the viruses themselves.