Organisms are generally designated as “extreme” when they possess adaptations that allow them to flourish when one or more environmental parameters (e.g., temperature, ionizing radiation, oxygen, water, nutrients, inorganic ions) are at unusually high or low levels. A trade-off for this extraordinary resilience is that such organisms often grow poorly (if at all) in more conventional milieus. As a result, few of them are mammalian pathogens. (Anaerobic bacteria, which frequently thrive in the hypoxic conditions afforded by devitalized tissue, are an exception to this rule.)
A broader definition of “extremity” encompasses all organisms that occupy narrow or unusual environmental niches. In this more inclusive scheme, a wealth of viruses, bacteria, fungi, and parasites with pathogenic potential in humans can be considered extreme. These organisms often pose formidable diagnostic challenges. Many have extremely narrow host ranges, rendering passage in animals and development of model systems for infection difficult or impossible.