Parasites and pathogens inherently cause harm to their hosts, but the degree to which that harm causes notable fitness costs for an individual host can be highly variable. While some effects of parasites on host fitness are drastic and immediate in nature, a growing body of evidence suggests that even parasites that cause chronic and subclinical infections can still have notable negative fitness consequences for hosts by reducing the probability of long-term survival and reproductive success. This chapter considers the ways in which fitness effects of infection are typically evaluated in birds and then examines the impacts of parasites and pathogens on diverse fitness-relevant traits in birds, including survival and components of reproductive success. Because fitness effects associated with infection can be highly variable and context-dependent, the chapter highlights some of the characteristics of parasites, hosts, or the environment likely to influence the extent of fitness effects of infection on avian hosts. The potential influence of abiotic factors such as climate and nutritional stress on the fitness impacts of parasites has not yet been well studied in birds but represent an important area for future research. Overall, understanding the individual-level fitness consequences of parasite infection is critical because these effects ultimately contribute to a population’s growth potential and determine the strength of selection that parasites place on host populations.