The fact that young organisms are more susceptible to infection and disease is not generally supported by evolutionary biology. On the contrary, given the strong pressure of natural selection on survival to reproductive age, it is expected that organisms' immune systems should be the most efficient and effective prior to reaching reproductive maturity, and subsequently decline. For example, children under 5 account for 11.5% of the world’s population but contribute approximately 55% of the annual global burden of disease in terms of disability-adjusted life years (Murray, 1996). As organisms age, their immune responses tend to decline, increasing the risk of infectious disease. Age-related immunosenescence is a term used to describe the decline in immune function that occurs with age, and it is a major factor in the increased risk of infection in the elderly.