Classical genetic techniques have made it possible in many instances to discern which bacterial factors are directly involved in causation of infection, as opposed to those that are associated with but do not directly contribute to virulence. By use of these methods as well as monoclonal antibodies, recombinant DNA, and other new techniques it has been shown that bacterial virulence is complicated, with involvement of many different bacterial factors at each step of infection; bacterial factors that facilitate one step of infection may actually impede a subsequent step. Interestingly, a large number of genes involved in toxin production or bacterial cell-surface structure are carried on unstable elements (phage, plasmids). In addition, many chromosomal genes affecting surface antigens or appendages involved in pathogenesis are subject to high-frequency variation, enabling the bacterium to adapt rapidly to different ecological niches or to evade host immunological defences. Genetic approaches have greatly increased our appreciation for the sophistication of successful bacterial pathogens, and are rapidly being used to create exciting new vaccines.