Geriatric infections: Decreased immunity or evolved opportunists?

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