The ability of bacteria, viruses, fungi and parasites to survive medicines that once killed them, making infections harder to treat; the WHO lists it among the top global public health threats.
Antibiotics transformed medicine in the twentieth century. Alexander Fleming discovered penicillin in 1928, and by the 1940s mass production had made it a life-saver. Fleming himself warned that misuse would lead to resistance, and within years resistant strains appeared. Microbes multiply fast and swap genes, so any use of antimicrobials selects for those that survive.
Resistance spreads within and between species. Bacteria can share resistance genes through plasmids (conjugation), take up DNA from the environment (transformation) or receive it via viruses (transduction). Overuse in humans, use as growth promoters in animals, poor sanitation, pharmaceutical effluents and weak infection control in hospitals have all accelerated the problem. The "New Delhi metallo-beta-lactamase" (NDM-1) gene, identified around 2008, became an international symbol of how quickly resistance can spread.
Methicillin-resistant Staphylococcus aureus (MRSA) bacteria, a colourised scanning electron micrograph from the US National Institute of Allergy and Infectious Diseases. Photo: NIAID, Public domain, via Wikimedia Commons.
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