(I) Bacteria are important pathogens in man. (II) A wide variety of them normally inhabit various parts of the human body, including the throat and the intestine. (III) Most of these residents are harmless bacteria and often prevent harmful microorganisms from flourishing. (IV) Pathogens can enter the body in food or air or through damaged skin. (V) Some, however, are opportunist pathogens and can cause disease when, for instance, one's immune system is compromised.