A pandemic is an occurrence of a disease that affects many people over a very wide area. (Collins English Dictionary)

Example:

COVID-19 pandemic is an opportunity to increase understanding of China

If there is an epidemic of a particular disease somewhere, it affects a very large number of people there and spreads quickly to other areas. (Collins English Dictionary)

Example:

Whether a disease is epidemic or endemic depends on the disease and the population.

If a disease or illness is endemic in a place, it is frequently found among the people who live there. (Collins English Dictionary)

Example:

Polio was then endemic among children my age.