The outcome of host/parasite relationships is not necessarily
predictable, nor does it inevitably result in coexistence
(commensalism)

> If the host is able to completely tolerate pathogen damage up to a certain replication rate, this may result in apparent commensalism, whereby infection causes no apparent virulence ... Tolerance tends to increase disease prevalence and may therefore lead to more, rather than less, disease-induced mortality. If the parasite is selected, even a highly efficient tolerance mechanism may result in more individuals in total dying from disease.

> To summarize the epidemiological effects, the evolution of host tolerance generally increases disease prevalence. Consequently, the number of pathogen-induced deaths may actually increase Complete tolerance potentially selects for a parasite strain that causes no virulence, although this is not always the outcome. the evolution of host tolerance may be partly responsible for the ubiquity of parasites in nature. Roy and Kirchner (2000) noted that because tolerance leads to higher prevalence of the parasite, it more easily allows their persistence. As demonstrated, commensal strains may have evolved higher replication and transmission rates. Intolerant hosts coming into contact with an evolved parasite would again experience high levels of virulence and transmissibility. The emergence of disease from seemingly commensal organisms may therefore occur without changes in the parasite but due to a lack of tolerance mechanisms in new host populations.

> Throughout our study, we have assumed only single infections. However, infected hosts may often harbor more than one parasite strain simultaneously.

THE EVOLUTION OF PARASITES IN RESPONSE TO TOLERANCE IN THEIR HOSTS:
GOOD, THE BAD, AND APPARENT COMMENSALISM
MARTIN R. MILLER, ANDREW WHITE AND MICHAEL BOOTS
Evolution, 60(5), 2006, pp. 945–956

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