When declaring a pandemic, the World Health Organization has the last word. There is no threshold, such as a certain number of deaths or infections, or a number of countries affected, that needs to be met. And once a pandemic is declared, it becomes more likely that community spread will eventually happen, and governments and health systems need to ensure they are prepared for that.
Epidemic is a term that is often broadly used to describe any problem that has grown out of control. An epidemic is defined as "an outbreak of a disease that occurs over a wide geographic area and affects an exceptionally high proportion of the population." An epidemic is an event in which a disease is actively spreading.
As humans have spread across the world, so have infectious diseases. Even in this modern era, outbreaks are nearly constant, though not every outbreak reaches pandemic level as the COVID-19 has.
The original use of the word Quarantine was the act of anchoring a ship arrived in Venice, Italy, for 40 days before landing.
Infectiousness of any disease is measured by the reproduction number (R0, or R naught). For example, Smallpox has an R0 of 6 whereas Measles has an R0 of 16.
Vaccination, if available, and herd immunity can curb the spread of disease.
Big cities, with exploding population and traffic, can lead to the rapid spread of any infectious disease.
Pandemic: It's a scary word. But the world has seen pandemics before, and worse ones, too. Consider the influenza pandemic of 1918, often referred to erroneously as the "Spanish flu." Misconceptions about it may be fueling unfounded fears about COVID-19, and now is an especially good time to correct them.
It is often referred to incorrectly as the “Spanish flu.” Between 50 and 100 million people are thought to have died, representing as much as 5% of the world’s population. Half a billion people...
The so-called Spanish flu did not originate in Spain. The geographic origin of the flu is debated to this day, though hypotheses have suggested East Asia, Europe, and even Kansas. The influenza pandemic from 1918 got this name most likely because of the WWI context: The major countries involved in the war were keen to avoid encouraging their enemies, so reports of the extent of the flu were suppressed in Germany, Austria, France, the United Kingdom and the U.S. By contrast, neutral Spain had no need to keep the flu under wraps. That created the false impression that Spain was bearing the brunt of the disease.
The 1918 flu spread rapidly, killing 25 million people in just the first six months. This led some to fear the end of mankind and that the whole thing was caused by a form of super-virus. Recent studies show that the high death rate can be attributed to crowding in military camps and urban environments, as well as poor nutrition and sanitation, which suffered during wartime.