Final answer:
An epidemic is a disease that occurs in an unusually high number of individuals in a population at the same time. A pandemic is a widespread epidemic that spreads across multiple populations. An endemic disease is a disease that is constantly present in a population.
Step-by-step explanation:
An epidemic is a disease that occurs in an unusually high number of individuals in a population at the same time, such as the flu.
A pandemic, on the other hand, is a widespread epidemic that spreads across multiple populations, often across continents or even worldwide, like HIV/AIDS.
An endemic disease is a disease that is constantly present in a population, usually at low incidence, such as malaria.