Some people do not stop and wash their hands after they've eaten. These same people cook your meals, touch the same doorhandles and also shake your hand.
If you have the sick habit of biting your fingernails, their shit has a route right into your mouth. Does it taste good to chew on shit? Do you think it's healthy? No.
SO ALWAYS WASH YOUR HANDS
AND DON'T BITE YOUR NAILS.