The past two months have thrust the terminology of public health into the forefront of people’s conversation. Accommodating themselves to life under social distancing guidelines due to the spread of the coronavirus, not a day goes by when someone doesn’t mention the word “pandemic.” But when did this epidemic turn into a pandemic? Why are people casually referring to this disease outbreak as a “plague?” Is there a difference between isolation and quarantine?

The National Interest has developed a terminological and etymological guide for readers to make sense of the situation.

An epidemic is the rapid spread of a disease among a given population in a short duration of time. The key to the definition is the size of the population and the area it encompasses. An epidemic is used as a description when the infected cover a significant local population, or even up to the size of a country. When the coronavirus began to develop in the Wuhan province of China in November and December 2019, it was considered an epidemic isolated to China.

When the virus spread to other countries in 2020, however, the epidemic became a pandemic. A pandemic is an epidemic that has spread to multiple countries, continents, and even worldwide. Specifically, pandemics are the spread of a disease, unlike seasonal flus which are not considered pandemics because they develop around the world simultaneously. The World Health Organization declared the coronavirus a pandemic on March 11.

The technical name for this specific outbreak is SARS-COV-2, and it is the first time a coronavirus has caused a pandemic. Coronavirus is a broader term that refers to a group of viruses, originating in animals, that cause respiratory tract infections.

The reason people colloquially call any outbreak of disease a “plague” is because the most impactful pandemic in history, the Black Death of the fifteenth century, was caused by the plague. The plague, referring to the specific medical disease, is caused by the bacterium Yersinia pestis.

There are three kinds of plague. The most famous is the bubonic plague, which is the same strain that formed the Black Death. Caused by fleas infected with the bacteria, and carried by rats, the disease is named after the buboes that form on victim’s bodies, growths that form around the neck and groin that represent lymph gland swelling.

The other two kinds of plague are pneumonic and septicemic. Pneumonic plague is when the bacteria becomes airborne and infects the lungs instead of the lymph glands. Septicemic plague is the rarest and deadliest; it is when plague has infected the blood directly, and in the Middle Ages it would almost guarantee death within twenty-four hours.

It was the Black Plague that gave us the word “quarantine,” derived from the Italian “quarantena,” meaning “forty days.” That is the length of time that ships in fifteenth century Venice would have to wait at port before coming ashore. This period was meant to forestall the spread of the plague, which was typically carried by the rats in trade ships.

Quarantine, the separation of a person or people who may be infected, is different than medical isolation, which is when the person or people separated are known to be infected and contagious. The two terms are not interchangeable.

Hunter DeRensis is a senior reporter for the National Interest. Follow him on Twitter @HunterDeRensis.

Image: Reuters.