A pandemic can occur when a new virus emerges and there is worldwide spread of the disease. Most people do not have immunity to a new virus. Viruses that have caused past pandemics usually come from animal viruses that have mutated to affect humans.
For a new virus to have pandemic potential it must meet three criteria:
-humans have little or no pre-existing immunity against the virus
-the virus causes disease in humans
-the virus can spread efficiently from person to person.
Previous pandemics include Spanish Influenza in 1918 or H1N1 Swine Flu in 2009. Only Type A influenza viruses have been known to cause influenza pandemics. This COVID-19 pandemic is the first caused by a coronavirus.
Source: https://www.health.nsw.gov.au/Infectious/alerts/Pages/coronavirus-faqs.aspx#sec8
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