Acquired resistance is:

A. Substance that can inhibit the growth of bacteria: This describes an antibiotic, not acquired resistance. B. Ability of an organism to cause disease in humans: This describes pathogenicity, not acquired resistance. C. The severity of a disease that an organism is able to cause: This describes virulence, not acquired resistance. D. When a microbe is no longer affected by a drug following treatment with an anti-infective: This is the correct definition of acquired resistance. Over time, microbes can develop mechanisms to evade the effects of antibiotics, rendering them ineffective.