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 improve d "fit" are seen from the perspective of the infectious disease sciences as ever-changing viral phenotypes and emerging disease risks. In the Darwinia n cause:effect equation, we can characterize very well the effects of mutat ion and selection - these are catalogued as new viral phenotypes or pathoty pes. However, the selective forces themselves driving such changes remain r ather mysterious. Many selective forces must be at work, acting on the viru s, 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 e nvironmental change, but usually it is clear that changes are driven by hum an activity. Most important must be the ever increasing density of human, d omestic animal and crop plant populations and the consequent increased oppo rtunities for transmission of viral variants. Also important must be the gr eat 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 aroun d the world, again favoring viral occupation of new niches. The rise of bio terrorism adds yet another threat. Increasing numbers of emerging viral dis ease episodes seem to be linked to a decline in global resources for proven public health programs, agricultural extension programs, and the like, pro grams that have stood in the way of the spread and evolution of viral patho gens. 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 level s rather than at the level of the viruses themselves.