epidemic of infectious disease that has spread across a large region with global health impact

A pandemic (from Greek πᾶν, pan, "all" and δῆμος, demos, "people") is an epidemic of an infectious disease that has spread across a large region, for instance multiple continents or worldwide, affecting a substantial number of people.


  • The connection between epidemics and pandemics and the growth of the modern state is clear. As early as the fifteenth century, in response to the plague, Italian city states formed state-sponsored boards of health. The cholera pandemics of the nineteenth century led to nationwide efforts at quarantine—efforts that could only be carried off by a central state. Measures such as compulsory vaccination also demonstrated this connection.
  • The R0 values have important implications for disease control. R0 magnitude indicates the level of mitigation efforts needed to bring an epidemic under control. Mitigation reduces the effective transmission coefficient, now called Re. Re needs to be reduced to less than 1 to ensure cessation of an epidemic, which can be done by rapid case identification, quarantine measures, and physical distancing to prevent secondary transmissions. For childhood diseases such as measles, the cessation of epidemic spread was achieved with an effective vaccine. However, a vaccine has never been a major tool for control of pandemics because they either occurred before the era of modern vaccines or, as in 2009, the vaccine became available only after the first waves had already occurred.

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