An epidemic is an outbreak of an infectious disease that spreads at a rate faster than expected. The term has its origin in the Greek language, "epi-" means upon and "demos" people. Epidemics are classified by their origin and transmission patterns. There can be a single, multiple, or continuous exposure to the source of the disease. The disease can also be transmitted by a vector (person to person) or through a common source.
On the other hand, a pandemic is an epidemic that spreads globally or across a large region. The term also has its origin in the Greek language, “pan” means all and "demos" people. According to the World Health Organization (WHO), a pandemic has the following characteristics:
- The emergence of a disease new to the population
- The agent infects humans, causing serious illness
- The agent spreads easily and sustainably among humans
The third criteria is especially useful for defining what a pandemic is because it excludes cancer and other non-infectious diseases even though they are widespread and cause serious illness.