thanks for the reply. What puzzles me is whyt this is different for humans. If I spent the day gardening, but didn't wash my hands all day, no harm would come to my skin (I know, I'm a gardener). If, however, I were to lick my hands clean every day I would almost certainly get ill. Why is this not the case for animals?

Somebody with more expertise in evolutionary biology will probably also contribute to this.

Humans live in a civilised society and hence take a shower and so have no need to lick themselves clean. While our tummies can handle a vast variety of substances the variety is probably not as wide as the tummies of cats or dogs can handle. It seems likely that as humans evolved from apes we lost the need and hence ability to lick ourselves clean without falling ill.

Also you don't eat raw food with your hands so the risk of illness is less. Similarly you don't run through woods naked thus picking up ticks and bites etc that then need to be removed.