The age distribution of infection is governed by the intrinsic transmissibility of the infection, by contact patterns between age groups, and by the age distribution of immunity from past infection. Thus, changes in age distribution of the population itself will impact upon the transmission dynamics of infection, perhaps in complex ways, and these in turn will impact upon the future age distribution of infection and disease and of susceptibility and immunity; the ''echoes'' of such perturbations in the dynamics of infection have the potential therefore to persist for a number of years (Williams and Manfredi, 2004). The processes of infection and disease are often strongly age related, so that changes in age at infection may have important consequences for the number of deaths or cases of serious disease. Well-known examples are rubella, mumps, and poliomyelitis in which infection of young children is likely to have few long-term consequences, but infection of young adults may have severe consequences, such as congenital rubella when infection occurs in the first trimester of pregnancy; meningitis or encephalitis, and orchitis in males, in the case of mumps; paralysis in the case of poliomyelitis (Mims et al., 2001). Age-related phenomena are found in many infections, affecting how the infection manifests itself in terms of symptoms, course, duration or outcome, and patterns of contact which vary by age also determine the likelihood of infection. In particular a number of infections manifest increasing severity with increasing age, so that in aging populations in which individuals may have not had the ''opportunity'' to be exposed to infection as a result of partial control (e.g., vaccination without elimination) they may run a greater risk of experiencing significant morbidity (Williams and Manfredi, 2004). Another zexample of age-related phenomena is responsiveness to immunization; for example, older individuals are less likely to generate a satisfactory immune response to hepatitis B vaccine. Such age-related processes can have substantial implications for the epidemiology of infectious disease, but often ones which are difficult to intuit (Medley et al., 2001). Modeling can help to elucidate the ways in which these processes may interact. Although simple compartmental models can be used to model in a simple way the population size effects of basic demographic processes of fertility, mortality and migration, to consider impacts of age-related processes of infection, models explicitly incorporating age structure and age-related processes are desirable. These are considered in the next section.
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