Final answer:
A video clip that becomes extremely popular quickly is said to have gone viral. When referring to diseases, a condition that is always present within a population is known as endemic, such as malaria in certain areas. The correct term for diseases that are always present is 'endemic'.
Step-by-step explanation:
The expression used when a video clip becomes very popular, accumulating thousands or millions of viewers in a short period, is viral. However, the question seems to be a mix-up with a term associated with diseases. For diseases, a condition that is constantly present in a population is labeled as endemic. For instance, malaria is endemic in certain tropical areas because it is consistently found there and affects many people.
An epidemic occurs when a disease spreads rapidly among many people in a short time in a particular area, while a pandemic is an epidemic that has spread over multiple countries or continents, usually affecting a large number of people. Re-emerging diseases are those that were previously under control but have begun to reappear as public health concerns.