Economics of infection at the extremes of age.

People are more vulnerable to infection at the extremes of age for a variety of reasons, the most important being that they are more likely to be in hospital in a crowded ward environment and to be at risk from hospital acquired infection. Recognition of this increased vulnerability to infection should be accompanied by equal emphasis on their increased susceptibility to nosocomial disease arising from the diagnosis or treatment of infection. An economic evaluation of infection at the extremes of age should include an assessment of need made in terms of the capacity of patients to benefit from investigation or treatment. Benefits should not be confused with treatment effects such as reduction in pyrexia or correction of other physiological abnormalities. Ideally benefits should be quantified in a manner which allows comparison with the cost-effectiveness of other uses of health care resources. In order to achieve this aim clinicians must understand the economic terms opportunity cost and marginal cost-effectiveness. These terms are defined in general terms and then applied to examples of investigation, prevention and treatment of infection at the extremes of age.