Drug resistance occurs when diseases stop responding to treatments that once worked. It arises through evolution: random genetic mutations give some microbes, cancer cells, or pests a survival advantage under drug pressure. These survivors reproduce, and over time the population becomes dominated by resistant forms.

In infectious diseases, resistance is driven by widespread and often inappropriate use of antibiotics, antivirals, antifungals and antiparasitic drugs. Bacteria can resist antibiotics by pumping drugs out, breaking them down, altering drug targets or forming protective biofilms. Resistance genes also spread horizontally between microbes. In viruses like HIV or influenza, high mutation rates under antiviral treatment rapidly select resistant variants. Fungi and parasites similarly evolve under antifungal and antiparasitic drugs, threatening treatments for diseases such as malaria.

Cancer drug resistance is a parallel problem. Tumors are genetically diverse; some cells inherently tolerate chemotherapy or targeted agents, or acquire resistance during treatment. Mechanisms include mutations in drug targets, activation of alternative signaling pathways, enhanced DNA repair and drug efflux pumps. The tumor environment, including low oxygen and protective stromal cells, can further shield cancer cells from drugs.

In agriculture, heavy reliance on pesticides and herbicides selects for resistant insects, weeds and plant pathogens, reducing crop yields and prompting use of higher doses or new chemicals. Across fields, key responses include using combination therapies, optimizing drug dosing, limiting unnecessary use, monitoring resistance patterns and developing new treatments. Understanding the evolutionary and molecular basis of resistance is central to designing more durable therapies and control strategies.