Parasites pose risks to host individuals, populations, and communities, though I argue that a majority of negative impacts are severely exacerbated by global change. A review of species extinctions caused by parasites or pathogens highlighted that disease was never the sole cause of extinctions but acted with other drivers such as habitat loss.

Parasites interact with global change drivers at multiple scales. For example, chronic digenean trematode infections in bivalve individuals frequently cause castration but do not typically increase mortality.

However, environmental anoxia increases death in bivalves infected with trematodes (but not uninfected bivalves), an effect that cannot be appreciated by considering either environmental change or parasitism in isolation. At a population level, global change may enhance parasite transmission between viable hosts.

Stressors such as temperature change can increase the survival time of trematode infective stages in the environment, while winter food shortages enhance the rate of viral shedding. Changes to host community structure alter transmission, which may enhance prevalence or intensity of parasites in given host species.

. .