Opportunistic infection

An opportunistic infection is an infection that takes hold mainly in people whose immune defences are impaired, and that tends to run a more severe course in them than it would in someone with a healthy immune system. The organisms involved may be viruses, bacteria, fungi or parasites. In a person with normal immunity such an organism would usually cause only a mild illness or none at all. Impairment can come from HIV and AIDS, from immunosuppressive drugs such as those given in cancer treatment, from a disturbed microbiome such as altered gut flora, or from a breach in the body's barriers such as penetrating trauma. Some of these pathogens, Clostridioides difficile among them, can live in healthy hosts without symptoms until the balance of the immune system is upset, and misuse or overuse of antibiotics can open the way to resistant strains. No single person is credited with discovering the category; it emerged through the work of many scientists as treatment advanced.