Host age structure reshapes parasite symbiosis: collaboration begets pathogens, competition begets virulent mutualists.

Background: Symbiotic relationships are ubiquitous in the biosphere. Inter-species symbiosis is impacted by intra-specific distinctions, in particular, those defined by the age structure of a population. Older individuals compete with younger individuals for resources despite being less likely to reproduce, diminishing the fitness of the population.