Drug resistance is the reduction in effectiveness of a drug in curing a disease or improving a patient's symptoms. When the drug is not intended to kill or inhibit a pathogen, then the term is equivalent to dosage failure or drug tolerance. More commonly, the term is used in the context of diseases caused by pathogens.
Pathogens are said to be drug-resistant when drugs meant to neutralize them have reduced effect. When an organism is resistant to more than one drug, it is said to be multidrug resistant.
Drug resistance is an example of evolution in microorganisms. Individuals that are not susceptible to the drug effects are capable of surviving drug treatment, and therefore have greater fitness than susceptible individuals. By the process of natural selection, drug resistant traits are selected for in subsequent offspring, resulting in a population that is drug resistant.
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