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An epidemic is a disease that spreads over a large area and affects many people at the same time. Epidemics are when a disease spreads from person to person faster than doctors can control. If the disease spreads over the whole world, sometimes it is called a pandemic.
Such diseases must be treated quickly and properly otherwise they can infect people on large scale and pose a danger to the world. It is very important that timely vaccination and treatment is provided. The study of epidemics is part of epidemiology.