When is an epidemic considered a pandemic, and what is the difference? What do health officials mean when they recommend “self-quarantining” or “social distancing”?

As the coronavirus spreads around the world, new terms are entering the lexicon — and we’re here to help. Here’s a guide to the words and phrases you need to know to keep informed of the latest developments.

PANDEMIC

The World Health Organization declared the coronavirus outbreak a pandemic after it spread across six continents and more than 100 countries. A pandemic is the worldwide spread of a new disease that affects large numbers of people. The WHO had avoided using the word before Wednesday because it didn’t want to give the impression that the disease was unstoppable.

“Pandemic is not a word to use lightly or carelessly,” Dr. Tedros Adhanom Ghebreyesus, the director general of the WHO, said at a news conference.

EPIDEMIC

An epidemic is a regional outbreak of an illness that spreads unexpectedly, according to the WHO. The Centers for Disease Control and Prevention defines it as an increase, often sudden, in the number of cases of a disease above normal expectations in a set population.

COVID-19

The technical name for the coronavirus is SARS-CoV-2. The respiratory disease it causes has been named the “coronavirus disease 2019,” or COVID-19.

Coronaviruses are named for the crown-like spikes that protrude from their surfaces, resembling the sun’s corona. Coronaviruses are among a large number of viruses that are common in people and many animals. The new virus, first detected in China, is believed to have originated in bats.

While antibiotics don’t work against viruses, researchers are testing drugs that could disrupt viral proteins and stop the infection.

FLATTENING THE CURVE

The term refers to a curve in a chart that shows when a surge of new coronavirus cases are expected to strike and illustrates why slowing the spread of the infection is nearly as important as stopping it.

An illustration by the visual-data journalist Rosamund Pearce, based on a graphic in a CDC paper titled, “Community Mitigation Guidelines to Prevent Pandemic Influenza,” showed what Drew Harris, a population health analyst at Thomas Jefferson University in Philadelphia, called two epi curves.

The high curve showed a peak indicating a wave of coronavirus outbreak in the near term; the other had a flatter slope, indicating a more gradual rate of infection over a longer period of time.

Slowing and spreading out the tidal wave of cases will save lives.

STATE OF EMERGENCY

A state of emergency can be declared during natural disasters, epidemics and other public health emergencies. Declaring a state of emergency, as more than a dozen states — including New York, New Jersey and Michigan — have done, gives government officials the authority to take extra measures to protect the public, such as suspending regulations or reallocating funds to mitigate the spread of a disease.

INCUBATION

The incubation period is the time it takes for symptoms to appear after a person is infected. This time can be critical for prevention and control, and it allows health officials to quarantine or observe people who may have been exposed to the virus.

The new coronavirus has an incubation period of two to 14 days, according to the CDC, with symptoms appearing about five days after infection in most cases.

During the incubation period, people may shed infectious virus particles before they exhibit symptoms, making it almost impossible to identify and isolate people who have the virus.

SOCIAL DISTANCING

The virus can easily spread in dense places — in a packed subway car, for example, or at a rally or concert.

Social distancing refers to measures that are taken to increase the physical space between people to slow the spread of the virus. Examples include working from home, school closings and the postponement or cancellation of mass gatherings, such as the South by Southwest music, technology and film conference.

By maintaining a distance of 6 feet from others when possible, people may limit the spread of the virus.

SELF-QUARANTINE

This is key to keeping the virus from spreading, along with measures like social distancing, frequent hand-washing and wearing masks.

While isolation refers to separating sick people from people who aren’t sick, quarantine refers to the separation and restriction of movement of people who were exposed to the virus to see if they become sick.

Who should self-quarantine? If you’ve left an area with widespread or continuing transmission, including China, Iran, Italy and South Korea, you should self-quarantine at home for a period of 14 days from the time you left, according to the CDC.

While in quarantine, you shouldn’t receive any visitors and must stay 3 to 6 feet from others at all times.

According to the CDC, once someone has been in isolation for 14 days and hasn’t become ill, he or she is not considered to be a risk to other people.

FATALITY RATE

The case fatality rate is the number of deaths divided by the total number of confirmed cases. Eventually, scientists hope to have a more comprehensive number called the infection fatality rate, which includes everyone who was infected with the virus.

The WHO estimates the fatality rate of the new coronavirus to be about 3%, based on current data, but experts suggest 1% is more realistic.

R-NAUGHT

The R-naught, or R0, is a virus’s basic reproductive number — an epidemiologic metric used to describe the contagiousness of infectious agents.

At its simplest, the basic reproductive number can show us how worried we should be about infection, according to Dr. Adam Kucharski, a mathematician at the London School of Hygiene and Tropical Medicine. If the R0 is above one, each case is expected to infect at least one other person on average, and the virus is likely to keep spreading. If it’s less than one, a group of infected people are less likely to spread the infection.

Research is still in its early stages, but some estimates suggest that each person with the new coronavirus could infect between two and four people.

CONTAINMENT

The virus’s high transmission rate has made it difficult to effectively contain the outbreak. Containment refers to the use of any available tools to mitigate the spread of a disease, said Adam Ratner, the director of the division of pediatric infectious diseases at NYU Langone Health.

Early on, the Trump administration sought to slow the spread of the virus by barring entry into the United States by any foreign nationals who had traveled to China in the previous 14 days, excluding the immediate family members of American citizens or permanent residents. While that measure may have bought the government time to prepare, the administration made key missteps in its efforts to make widespread testing available in the early days of the outbreak, when containment would have been easier.

Ratner said the coronavirus is particularly hard to contain because it is “reasonably transmissible” and some people who don’t have a lot of symptoms can still pass the virus to others. “That’s been part of the problem,” he said, “but it also points to the fact to how interconnected we all are and how quickly this thing spread from Asia to the rest of the world.”

© 2020 The New York Times Company