Can evolution explain why the young are often more susceptible than adults to infection?
(PHYSorg.com Biology)

In many species, including humans, the young are often more susceptible to infection than adults, even after accounting for prior exposure to infection. From an evolutionary perspective this may seem puzzling, as dying young or becoming infertile due to infection means organisms will be unable to reproduce. However, new research from the University of Bath suggests that many species may have evolved to prioritise growth over immunity while maturing.