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from NPR, February 14, 2017 by Natalie Jacewicz; image by Katherine Du/NPR The world is in a hyperinfectious era. And that means there are a lot of words being tossed around that you might not be familiar with. Or maybe you have a general idea of what they mean but wish you knew more. Here are some key terms and definitions. And yes, there will be a quiz (coming in March so you have time to study). Epidemic: A sudden increase in the number of cases of a disease in a particular geographic area, beyond the number health officials typically expect. An increase that occurs in a relatively small geographic area or among a small group of people may be called an "outbreak." For example, the Centers for Disease Control and Prevention calls HIV/AIDS, which affects 1.2 million people in the United States, an " epidemic." By contrast, the CDC called two cases of sickness from drinking raw milk ( listeriosis) in the United States an " outbreak." Pandemic: An epidemic spanning many countries and/or several continents. The difference between an outbreak, an epidemic and a pandemic can be murky and depends on the opinions of scientists and health officials. Emerging disease: A disease that occurs in the population of a certain geographic region for the first time, or a disease that's been present at low levels in a region but then rapidly reaches new peaks in the number of cases reported. Animal-human interface: The points of contact between animals and humans — when people cut down forests and set up dwellings where forest animals are still prevalent, for example. Some types of diseases spread from animals to humans at this interface. (Note: In all these definitions "animal" refers to nonhuman animals.) Reservoir: An animal, plant or environment in which a disease can persist for long periods of time. For example, some bats serve as a reservoir for rabies and can spread the disease by biting humans. But the bats — and other reservoir species — may not experience symptoms because of built-in immunity. A disease reservoir is analogous to a water reservoir. But instead of supplying water, a disease reservoir serves as a supply for a virus or other pathogen. Vector: Any living creature that can pass an infection to another living creature. Humans are technically vectors, but the term is more commonly applied to nonhuman organisms. Spillover: The transmission of a disease from one species to another. Sometimes a disease may reside in a plant or animal or even in soil,and then spread to humans. This spread of disease is called a "spillover event." Index case: The first case of a disease known to health officials. Some epidemiologists may refer to an index case as "patient zero." Read the entire list on NPR