pandemic is the worldwide spread of a new disease. An influenza pandemic occurs when a new influenza virus emerges and spreads around the world, and most people do not have immunity. Viruses that have caused past pandemics typically originated from animal influenza viruses
Usually, <em>a disease is considered pandemic</em> when it crosses international borders. A disease that does not cross international borders is an epidemic. An example of a pandemic would be the Black Plague (also called the Black Death or the Bubonic Plague), which <em>killed most of the population of Western Medieval Europe.</em>
