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The Evolution of Disease Organisms

What makes a successful pathogen? Evolutionary theory suggests that microbes that cause disease (pathogens) would evolve to a level of virulence that maximizes their transmission from one host to the next. However, some spectacularly nasty human pathogens, flesh-eating bacteria come to mind, do not fit the expected pattern. The human body is home to many harmless bacteria that make a living on us without causing disease. Biologists call that kind of relationship commensalism; for example, E. Coli lives in our intestines, and various kinds of staph live in the nooks and crannies of the human body, usually without causing disease. These commensal bugs can cause serious disease however if they breach the skin and get into the blood. Staph infections are of particular concern in hospitals because even in the cleanest environment everyone has them on their skin. People in hospital are already stressed by whatever put them there, and they often have IV lines and other procedures that break the skin barrier.

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