They are, according to Health.com:
- Sporadic, or infrequently occurring disease.
- Endemic or usual prevalence of an illness.
- Epidemic or sudden increase of an illness or higher numbers of sick patients than expected.
- Pandemic or an epidemic that has spread to other countries.
The WHO will declare the illness as a pandemic using various models. But there is no one number that changes an epidemic or outbreak into a pandemic, The Guardian reported.
Why does it spread so far?
Most people don't have immunity when an outbreak spreads worldwide. They also have different epidemiological patterns. For example, while the normal flu hits during the winter months, the H1N1 pandemic happened in the summer, the WHO said.
How fast can a pandemic spread?
A virus can travel from a remote village to cities on all continents in 36 hours, The Centers for Disease Control and Prevention said.
How can you stop a pandemic?
CDC global health security experts work with other countries to help stop the spread by detecting and reporting cases, identifying the cause of illnesses, containing outbreaks and coordinating a response.