Micro-evolution and emergence of pathogens.

Changes in the epidemiology of infectious diseases are the direct result of ecological and evolutionary changes in hosts and parasites. Precisely what the causal processes are is rarely known in any particular case, and this hinders the design of appropriate control strategies. This is particularly so for emerging infections, as opportunity is rapidly lost to study the ecological parameters which might have affected initial emergence. However, molecular evolutionary studies of the pathogens can yield data which discriminate between possible causes. The current distribution of DNA sequence variation is important information which may reveal past and current changes in pathogen population structures, and can also identify adaptive changes in pathogen genes which have affected their evolution. Such studies have been quite intensively performed on particular viral and bacterial pathogens, and some of the successes of these are noted here. Approaches to understanding the recent evolution of eukaryotic pathogens are outlined, with particular reference to current problems of emerging zoonoses, and changes in virulence and drug resistance.