Pathogen dynamics across the diversity of ageing

Reproduction, mortality and immune function often change with age, but do not invariably deteriorate. Across the tree of life, there is extensive variation in age-specific performance and changes to key life-history traits. These changes occur on a spectrum from classic senescence, where performance declines with age, to juvenescence, where performance improves with age. Reproduction, mortality and immune function are also important factors influencing the spread of infectious disease, yet there exists no comprehensive investigation into how the ageing spectrum of these traits impacts epidemics. We used a model laboratory infection system to compile an ageing profile of a single organism, including traits directly linked to pathogen susceptibility, and those that should indirectly alter pathogen transmission by influencing demography. We then developed generalisable epidemiological models demonstrating that different patterns of ageing produce dramatically different transmission landscapes: in many cases ageing can reduce the probability of epidemics, but it can also promote severity. This work provides context and tools for use across taxa by empiricists, demographers and epidemiologists, advancing our ability to accurately predict factors contributing to epidemics, or the potential repercussions of senescence manipulation.