A disease that occurs regularly in a particular area, as malaria does in many tropical countries, is said to be endemic. The word endemic, built from the prefix en-, in or within, and the Greek word demos, people, means within the people (of a region). A disease that affects many more people than usual in a particular area or that spreads into regions in which it does not usually occur is said to be epidemic. This word, built from the prefix epi-, meaning upon, and demos, means upon the people. In order for a disease to become epidemic it must be highly contagious, that is, easily spread through a population. Influenza has been the cause of many epidemics throughout history. Epidemics of waterborne diseases such as cholera often occur after natural disasters such as earthquakes and severe storms that disrupt or destroy sanitation systems and supplies of fresh water.

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