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Virulence (the tendency of a pathogen to reduce a host's fitness) evolves when a pathogen can spread from a diseased host, despite the host becoming debilitated. Horizontal transmission occurs between hosts of the same species, in contrast to vertical transmission, which tends to evolve toward symbiosis (after a period of high morbidity and mortality in the population) by linking the pathogen's evolutionary success to the evolutionary success of the host organism. Evolutionary biology proposes that many pathogens evolve an optimal virulence at which the fitness gained by increased replication rates is balanced by trade-offs in reduced transmission, but the exact mechanisms underlying these relationships remain controversial.