Evolved resistance to endemic microparasites may reduce susceptibilities, but as barriers to microparasite distributions are weakened, and environments become more stressful, unexposed populations may be impacted and pathogenicity enhanced. Instead, moderately virulent microparasites persisting for long periods across multiple environments are of greatest concern. Microparasites selected for enhanced virulence in culture settings should be less successful maintaining infectivity in wild populations, as once the host dies, there are limited opportunities to infect new individuals. Emerging diseases are impacting animals under high-density culture, yet few studies assess their importance to wild populations.
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