Pathogens are potent selective forces that can reduce the fitness of their hosts. While studies of the short-term energetic costs of infections are accumulating, the long-term costs have only just started to be investigated. Such delayed costs may, at least in part, be mediated by telomere erosion. This hypothesis is supported by experimental investigations conducted on laboratory animals which show that infection accelerates telomere erosion in immune cells. However, the generalizability of such findings to natural animal populations and to humans remains debatable. First, laboratory animals typically display long telomeres relative to their wild counterparts. Second, unlike humans and most wild animals, laboratory small-bodied mammals are capable of telomerase-based telomere maintenance throughout life. Third, the effect of infections on telomere shortening and ageing has only been studied using single pathogen infections, yet hosts are often simultaneously confronted with a range of pathogens in the wild. Thus, the cost of an infection in terms of telomere-shortening-related ageing in natural animal populations is likely to be strongly underestimated. Here, we discuss how investigations into the links between infection, immune response and tissue ageing are now required to improve our understanding of the long-term impact of disease.