Drug resistance (such as antibiotic resistance) occurs when disease-causing organisms change over time and adapt in ways that allow them to survive exposure to a medicine that in the past killed or controlled them.

Many kinds of bacteria have become resistant to common antibiotics designed to kill them. These are called antibiotic-resistant bacteria. For example, several strains of the bacteria that cause tuberculosis and sinus, ear, and lung infections can no longer be killed by many antibiotics. Other illnesses, such as malaria and gonorrhea, also have drug-resistant strains of organisms.

Frequent and/or inappropriate use of antibiotics leads to the development of drug-resistant bacteria. For example, taking antibiotics when they may not be needed, using them for a non-bacterial infection (such as a cold), or not taking all of an antibiotic as directed may promote the development of antibiotic-resistant strains of bacteria.