Why? Because a principle way our bodies fight viruses is by killing our own (infected) cells. When you kill large numbers of cells in a young child, that child

Why? Because a principle way our bodies fight viruses is by killing our own (infected) cells. When you kill large numbers of cells in a young child, that child won't grow to the same height, won't have the same IQ, and won't live as long. There are interesting radiological data for comparison. We should have prioritized vaccinating children, but didn't. This is a likely source of a lot of (perhaps most of) the Flynn effect over the last century and more.