playing outside with a fever: why was this a no-no?

[julen]

When we were small, my sisters and I were always kept inside when we had a temperature, even when we didn't feel that ill at all anymore. Friends' mums did the same: no playing outside until temperature was gone completely. I don't have a clue why this was so - can't really see any harm when my children, while a bit warm, feel well enough to want to play outside. Does anyone know a reason for this?!?