Comparative outcomes of parasite and pathogen infections. A, B. Host fitness is reduced and the host immune system is challenged. C, E. The immune system overcomes infection, and the host survives. D. The host, which has become more susceptible to infection because of a weakened immune system, is ultimately killed by a virulent or an opportunistic microbial pathogen. F. A high degree of virulence and host specificity leads to death of the host. Note that the terms parasite and pathogen are used interchangeably in epidemiological and ecological studies.