It depends on the environment. Mortality of cholera is high (~ 60 % if untreated), but by spreading abundantly the bacterium finds a supply of new victims. I s

It depends on the environment. Mortality of cholera is high (~ 60 % if untreated), but by spreading abundantly the bacterium finds a supply of new victims. I seem to remember a study from ages ago where they compared strains of HIV from South Africa and Japan. Condom usage in South Africa is low, so the virus evolved to develop a high viral load quickly. In Japan the opposite was true, condom usage was high, the strains that caused the victim to die early became extinct.