An epidemic occurs when an infectious disease spreads quickly and affects many people at once, while a pandemic is a global outbreak of disease that kills millions of people. For example, an epidemic of SARs killed 800 people in 2003, while the Spanish flu pandemic killed as many as 50 million people in 1918.
Other examples of pandemics include the Asian influenza pandemic that killed 2 million people in 1957 and the Hong Kong flu pandemic that took the lives of 1 million people in 1968. A more recent pandemic occurred in 2009-2010 when the H1N1 swine flu began to spread around the globe. The United States Centers for Disease Control and Prevention estimates that as many as 89 million people contracted this flu strain from April 2009 through April 2010, with as many as 18,300 deaths related to the flu. People in 74 countries were affected.
Pandemics generally occur with the development of a new sub type of virus to which the population has little or even zero immunity. Most of the viruses that become pandemics spread from one person to another by coughing or sneezing. Within 6 to 9 months, these pandemic viruses generally reach all parts of the world, helped along by air travel.
Pandemics are classified by phase by the World Health Organization, with the first phase being a virus in animals that causes no infections in humans. In phase 2, viruses cross over from animals to humans, and in phase 3, sporadic cases or clusters occur. By contrast, stage 6 pandemics feature community level outbreaks.