When it has been established that there has been an outbreak of disease, the very first step in an epidemiologist’s investigation is to collect preliminary data about the epidemic, to get a better idea of the “who”, “where”, and “when” of the situation. This is called descriptive epidemiology, because it involves describing the situation to better understand what is going on.
For example- suppose there is a sudden outbreak of flu. The first step that epidemiologists would take would be to use descriptive epidemiology. They would need to find out who has the flu, where do the people with the flu live, and when did people get infected with the flu. Once they have collected data answering these three questions, epidemiologists can get some clues about what to do next, because data that is collected may reveal trends that may show how or why the epidemic occurs.
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