ABSTRACT
In host–parasite co-evolution, parasites are assumed to have an advantage owing to their shorter generationtime. Evolution of pathogens within the lifetime of a host individual is implicated as a strong selective force inthe evolution of sex and aging in the host. However, this assumption or its testable predictions have not beenexamined empirically. We classified infectious bacteria and viruses into those that can have continued longterm existence on the host body (group 1) versus those that have only a short-term interaction during an activeinfection (group 2). We surveyed the literature for age-specific incidence data about infections from both thegroups. The age trends of the two groups show contrasting patterns. The incidence of infections by all group 1pathogens showed a 2.28- to 28-fold increase in older ages. In group 2, 6 out of the 9 pathogens showed asignificant declining trend in incidence with age. In both groups, there was greater mortality or morbidityamong the infected in the old-age classes. These patterns are better explained by pathogen evolution than byage-related decline in immunity