The fact that the young are often more susceptible to infection than adults cannot be explained by evolution because increased resistance to infection is likely to be favoured by natural selection. As a result, we would expect young animals to be relatively resistant to infection.

Susceptibility to infection varies with age mostly because of differences in immune experience and physiology.

- Newborns usually enjoy some passive immunity derived from their mother’s antibodies. However, these become less effective with time. In parallel, most young animals gradually build up their own active immunity as a result of contact with pathogens in the environment.

- In addition to the developing immune system, other physiological differences influence the age-related risk of infection. For example, in humans the gut is sterile at birth, but is soon colonized by bacteria that contribute to the barrier to infection. Similarly, the skin of newborns is thinner than that of adults and provides a less effective barrier.

- Finally, the effects of infection also tend to be more serious in the young. This is particularly noticeable with respiratory infections such as influenza and pneumonia, which can be fatal to infants.