Summary: Resistance is a natural response of pathogen populations to antibiotic treatment. Changes in pathogen DNA can affect genes involved in drug uptake, drug efflux, drug inactivation, and drug-target interactions. Some changes occur spontaneously; others are induced as a response to the stress of antibiotic treatment. Still others involve the movement of whole resistance genes from one microbe to another. The enormous size of pathogen populations makes even rare events noticeable when antibiotic pressure is applied. Although many mutations are probably harmful to the organism, those that confer antibiotic resistance can be the difference between life and death for the pathogen. Consequently, they are tolerated. Subsequent mutations may then i....fitnessmutant selection window hypothesisresistance mutations