This paper investigates simplified models of multiple infection. Its first part deals with superinfection: the more virulent strain quickly outcompetes its rivals. The second part deals with coinfection: the rate of new infections produced by one strain is unaffected by the presence of other strains. The two cases differ in expectations for the resultant range of strains within the host population; they are similar in that both predict a considerable increase in virulence based on optimizing basic reproduction ratio of the pathogen do not work if several strains of pathogens compete within the host
