A healthy person lives in harmony with the microbial flora that helps protect its host from invasion by pathogens, usually defined as microorganisms that have the capacity to cause disease. The microbial flora is mostly bacteria and fungi and includes normal resident flora, which is present consistently and which promptly reestablishes itself if disturbed, and transient flora, which may colonize the host for hours to weeks but does not permanently establish itself. Organisms that are normal flora can occasionally cause disease, especially when defenses are disrupted. Pathogens must be proficient at sensing their surroundings and responding to cues that promote survival in the face of a changing environment. Infectious agents are expert cell biologists, having evolved very clever strategies for exploiting specific molecular processes that contribute to cell integrity and organism homeostasis.