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COVID-19 Glossary

From airborne to zoonotic: a COVID-19 glossary

Living through the COVID-19 pandemic means dealing with a new and sometimes confusing vocabulary. Here is a guide to some of the most common words and phrases you might come across.

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Airborne transmission: When germs stay in the air for minutes to hours after activities such as breathing, talking, coughing, and sneezing, that is called airborne transmission. COVID-19 can spread easily indoors in this way.

Antigen test (also known as home test, rapid test, and self test): Antigen self-tests, sometimes called a “rapid test” or “home test,” detect virus proteins in the body. Antigen self-tests use saliva and nasal swab samples. Results take 15-30 minutes. These tests are more likely than PCR tests to give a false-positive or false-negative test result.

Asymptomatic: Not having signs of illness (symptoms). People who are asymptomatic can still be infected with COVID-19 and can spread the virus to others.

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Bivalent vaccine: A bivalent vaccine targets two strains of a virus. For example, the updated booster vaccines from Moderna and Pfizer in Fall 2022 target the Omicron variants BA.4 and BA.5 as well as the original strain of COVID-19.

Booster: A booster is an additional dose of COVID-19 vaccine that increases the strength and the duration of protection. Boosters stimulate your body’s immune system when the first vaccinations fade over time. We get boosters with other common vaccines like tetanus.

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Case fatality rate (CFR): The number of deaths divided by the total number of known cases of a disease. For example, if 100 people in a community are diagnosed with COVID-19 and two of them die, the case fatality rate is 2/100, or 2%.

It's important to keep in mind that the CFR often overestimates the actual mortality (death) rate, and can make a disease seem more deadly than it is. That’s because some people never get diagnosed, so we don't always have an accurate count of the cases.

Close contact: A close contact is someone who is less than 6 feet away from an infected person for 15 minutes or more over a 24-hour period.

Cluster: A collection of cases happening in the same place around the same time, above what is normal. For example, you might see a cluster of cases in a long-term care facility, a workplace, or among people who attended a party.

Community transmission: When a contagious disease is spreading in a community but we don’t know how or where every infected person got exposed.

Confirmed case: A person (with or without symptoms) who received a positive result from a COVID-19 laboratory test.

Contact tracing: Identifying all the people who may have been in close contact with an infected person while they were contagious.

Coronavirus: A family of viruses. Some coronaviruses cause the common cold, and others cause serious health issues, like pneumonia. Coronaviruses start in animals, like camels, civets and bats. Most of the time, people can’t catch coronavirus. But once in a while, a coronavirus can spread from animals to people and then from person to person.

COVID-19: A shorthand for "coronavirus disease 2019."

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Endemic: A disease that is consistently present over time. This makes the disease spread rates predictable, as they are with the common cold and flu.

Epidemic: An increase in the number of cases of a disease in a population, above what is normal. Epidemics often happen suddenly. COVID-19 began as an epidemic in China.

Epidemiology: The study of the spread or pattern of sickness in a group of people.

Exposed: You are considered exposed to COVID-19 if you have been less than 6 feet away from an infected person for 15 minutes or more over a 24-hour period.

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Flattening the curve: Slowing the spread of a disease so that you don’t overwhelm the healthcare system. The “curve” represents the number of cases over time. When you flatten the curve, you prevent a big spike of new cases in a short period of time. Instead, you’ll probably see the same number of cases spread out over a longer period of time.

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Hand hygiene: Washing hands with clean, running water and soap for at least 20 seconds. Or using hand sanitizer that contains at least 60% alcohol. Hand hygiene is a key strategy for slowing the spread of COVID-19 and many other diseases.

Herd immunity: When large numbers of people in a population have immunity (or protection) against a virus through vaccination or previous infection. Herd immunity makes it more difficult for diseases to spread between people.

High-risk populations: Certain groups of people who are more likely to get seriously ill from COVID-19. People over 60, people with underlying health issues, and pregnant people may be at higher risk.

High-quality mask: Wear the best quality mask you can get. Higher quality masks are better at filtering out the virus. Examples, in order of quality are:

  • Certified N95, KN95, or KF94 masks; or
  • Surgical masks; or
  • Cloth masks with multiple layers of breathable, tightly-woven fabric.

For more information on effective masks, see “Well-fitting / snug-fitting mask” below.

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Immunity: Protection from disease. If you have immunity to a disease, you won’t get sick from it.

Immunocompromised: Having a reduced ability to fight infections and other diseases. People can be immunocompromised because of certain conditions, like AIDS, cancer, and diabetes, or because of certain medications and treatments, like chemotherapy. People who are immunocompromised may be more likely to get very sick from COVID-19.

Incubation period: The time between when a person first gets exposed to a germ and when they develop symptoms. For COVID-19, the incubation period ranges from 2 to 14 days.

Isolation: Isolation separates sick people with a contagious disease from people who are not sick.

See the Isolation and quarantine page for more details.

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Long COVID: A condition after COVID-19 infection where symptoms such as fatigue, cough, shortness of breath, headache, or brain fog can last for weeks or months. Long COVID symptoms can happen to anyone who has been infected, even if the illness was mild or they didn’t have any symptoms in the first days or weeks of their infection.

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Mandate: A policy requiring certain actions, put in place by the local, state, or federal government. When actions are mandated, they are legally required and not optional.

Molecular/NAAT test: Tests that detect if COVID-19 genetic material is in the body. These tests are done on samples collected via a nasal swab (from the nose). These tests include PCR and TMA.

Monovalent vaccine: A monovalent vaccine targets only one strain of a virus.

mRNA vaccine: Messenger RNA (mRNA) vaccines help our bodies develop immunity to the virus that causes COVID-19 without us having to get the illness. mRNA vaccines (like the Pfizer and Moderna vaccines) protect us by instructing our own cells to produce a protein that can teach the immune system to prevent COVID-19 illness without using any part of the COVID-19 virus at all. mRNA vaccines have been researched for many years and have been used to fight Ebola and other diseases.

Multigenerational household: A home where more than two generations are living under the same roof. This often includes households with a grandparent and at least one other generation.

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Novel coronavirus: A coronavirus strain that we haven't seen before. COVID-19 is novel because it is a new respiratory virus that was first identified in Wuhan, China in December 2019. Health experts are concerned because this new virus can cause severe illness and death in some people.

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Outbreak: Same definition as epidemic, but used for a smaller geographic area. For example, King County’s COVID-19 outbreak began in late February.

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Pandemic: An epidemic that has spread over several countries or continents. Pandemics usually affect a large number of people. It’s important to remember that the word pandemic is mostly about how much a disease has spread, not how dangerous the disease is. COVID-19 is now a pandemic because many countries throughout the world have cases.

PCR test: A PCR test is a type of molecular/NAAT test. These tests detect if COVID-19 genetic material is in the body. These tests are done on samples collected via a nasal swab (from the nose).

Personal protective equipment (PPE): Special clothing or equipment to protect people from infection. PPE can include gloves, gowns, aprons, masks, respirators, goggles, and face shields. It’s especially important for healthcare providers and emergency responders to have PPE.

Pneumonia: An infection of the lungs that can make it harder to breath. Pneumonia can range from mild to life-threatening.

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Quarantine: Quarantine separates and restricts the movement of people who were exposed to a contagious disease to see if they become sick.

See the Isolation and quarantine page for more details.

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Reproductive rate: Also called the R0, pronounced "R naught": A mathematical term that can indicate how quickly a disease is spreading. R0 tells you the average number of susceptible (unvaccinated, not protected) people who will catch a disease from one contagious person. If the R0 is 2, then on average, every case will create two new cases.

It's important to remember that the R0 can change over time and place. It depends on a lot of different factors, like environmental conditions and people's behaviors. For example, when people practice social distancing, the R0 may decrease.

Respiratory droplet: The spray made when a person sneezes, coughs, or even talks. In general, respiratory droplets can travel up to six feet before dropping to the ground or a nearby surface.

Respiratory illness: An illness that affects the parts of the body involved with breathing, like the nose, throat, and lungs.

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SARS-CoV-2: The specific coronavirus that causes COVID-19 disease.

Screening: A health check to see whether a person has a disease. You can screen by taking a person’s temperature and asking questions about symptoms.

Self-monitor: Checking yourself for symptoms. This includes regularly checking your temperature (typically, twice a day) and watching for signs of a respiratory illness, like fever, cough, or shortness of breath.

Serial testing: Testing that is repeated at different points in time is referred to as serial testing. Some self-tests are designed to be used in a series 24 to 48 hours apart. Serial testing may be more likely to detect infection among close contacts of a COVID-19 case than testing done at a single point in time.

Social distancing / physical distancing: Increasing the physical space between people to avoid spreading illness. Right now, we can practice social distancing by staying home as much as possible and keeping at least six feet away from other people when we need to go out.

Surge: An increase in the number of COVID-19 related cases, hospitalizations, or deaths. During a surge, use multiple layers of protection like masks, social distancing, and improved air ventilation indoors to keep safe. Stay up to date on vaccination – it is one of the most important things we can do to prevent surges.

Surveillance: Collecting, analyzing, and interpreting health-related data. Surveillance helps public health professionals plan, carry out, and evaluate their work.

Suspected case: A person that shows symptoms of COVID-19 but either has not been tested or is waiting for test results. If test results are positive, a suspected case becomes a confirmed case.

Swab: A swab is a small stick with a soft end. It looks like a long Q-tip. During a COVID-19 test, a nasal swab is placed gently inside of the nostril and swirled around for a few seconds.

Symptomatic: Having signs of illness (symptoms). The most common COVID-19 symptoms are fever, cough, and shortness of breath.

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Transmission: How a disease moves around, or spreads. Diseases can be transmitted in a few different ways. Some of these include:

  • Direct contact: when a disease spreads through skin-to-skin contact, kissing, and sexual intercourse.
  • Droplet transmission:
  • When germs stay in the air for minutes to hours after activities such as breathing, talking, coughing, and sneezing, that is called airborne transmission. COVID-19 can spread easily indoors in this way.

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Up-to-date (with your COVID-19 vaccines): You are considered up-to-date with your COVID-19 vaccines when you have completed your initial series of vaccinations and all recommended booster shots that you are eligible for. When you are up-to-date with your vaccines, you get the maximum protection available. Some people with weakened immune systems will need more doses to be up-to-date.

Read the latest information and updates from the CDC.

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Vaccine: The COVID-19 vaccines help our bodies develop immunity to the virus that causes COVID-19. When you get the vaccine, your body develops antibodies and other infection-fighting cells that protect you from severe illness, hospitalization, and death in case you are infected with the virus.

Variant: Viruses like COVID-19 change over time. The changed versions are called “variants.” COVID-19 variants are labeled using Greek letters such as “Delta” and “Omicron.” The CDC labels them as “variants of concern” when they spread more easily, can cause worse illness, reduce protection from vaccines, reduce the benefit of treatments, or reduce the ability of tests to detect the disease. Experts closely monitor variants of concerns for surges, severity of symptoms, and treatment. A “subvariant” is closely related to a known variant.

Ventilation: COVID-19 spreads through the air (see “Airborne transmission” above), so having good indoor ventilation, or air flow, is very important to prevent the spread. Ways to improve air flow include opening windows and doors, using a Heating, Ventilation, and Cooling (HVAC) system with high efficiency filters, using portable air cleaners, or a box fan with a MERV 13 furnace filter attached to the back.

Ventilator: A machine that helps patients breath when their lungs aren’t working well and they can’t get enough oxygen on their own.

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Well-fitting / Snug-fitting mask: A well-fitting mask covers your nose and chin, without gaps around the face. Masks that are loose, with gaps around your face or nose, are not as helpful in protecting you or others.

For more information on effective masks, see “High-quality mask” above.

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Zoonotic: A disease that spreads between animals and people. Common examples include rabies, Lyme disease, and West Nile virus.

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