Another consequence of the dramatically increased densities of humans, host reservoirs, and vectors is the increased number of pathogen genomes. The resulting increased levels of genetic variability can accelerate microbial adaptation, including evolution of pathogenesis, and antimicrobial resistance. Genetic variability increases with population size and density through a variety mechanisms including mutation. The probability of producing more virulent variants not only increases with host population size but also with crowding and co-mingling of different host species (31) . In general, parasite (pathogen)-host relations naturally constitute a co-adaptive/ evolutionary 'dance' along the pathogenicity threshold, which is likely to be crossed with greater frequency due to unnatural anthropogenic disturbances (32) independent of increasing population sizes and pathogen genetic diversity.