A single random gene mutation can have a large impact on an organism's disease-causing properties . A mutation that helps a microbe survive in the presence of an antimicrobial agent will quickly become predominant throughout the microbial population . Microbes also commonly acquire genes, including those encoding for resistance, by direct transfer from members of their own species or from unrelated microbes. Once established in a microbial population, resistance is virtually impossible to eradicate. The innate adaptability of microbes is accelerated by the selective pressure of widespread and often inappropriate use of antimicrobial agents.
Hospitals provide a fertile environment for drug-resistant pathogens.
For example, strains of HIV resistant to multiple antiretroviral drugs are now commonplace, and can be transmitted from an infected individual to an uninfected one. Although treatments that combine new drugs called protease inhibitors with other anti-HIV medications often effectively suppress HIV production in infected individuals, studies suggest that many treatment failures occur due to the development of resistance by the virus. Fungal pathogens account for a growing proportion of nosocomial infections, and clinicians are concerned that the increasing use of antifungal drugs will lead to drug-resistant fungi. Recent studies have documented resistance of Candida species to fluconazole, a drug used widely to treat patients with systemic fungal diseases. Parasitic diseases such as malaria are also becoming more difficult to treat.