I don't think we even want to know the dirtiest places to pick up germs. If we did, nobody would leave their own house.
To me, the worst thing in the supermarket is the buggy handles. Think about it. All those little drooling, slobbering kids - germ-carriers all - leaking all that bacteria all over where our hands must rest. ACK!
That's why I carry a little bottle of antibacterial gel everywhere I go. At least I'm not to the point where I wear medical gloves and a face mask out in public. Yet.