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There are a variety of terms that are used in discussions of the novel coronavirus, SARS-CoV-2019, and COVID-19.

Airborne/aersolized transmission spread

Asymptomatic carrier

Case fatality rate (CFR)

Droplet transmission/spread

Infection-to-fatality rate (IFR)

Infectious dose

Isolation

Multisystem inflammatory syndrome (MIS-C)

Presumptive positive

Positivity rate

Quarantine

Serologic test

Symptomatic case-fatality rate (sCFR)

Viral load

Zoonotic


Airborne/aersolized transmission spread

There is a lot of controversy among scientists about this definition, hence the public health debate about how SARS-CoV-2, the virus that causes COVID-19, spreads. Generally, what is meant is spread of a contagious disease from a respiratory droplet that is small enough to float in the air or spread farther than 6 feet. The spread of these droplets may be affected by humidity, air flow, and temperature, but more research in this area needs to be conducted. What isn’t clear is if these small droplets contain enough virus to cause someone else to become infected.

Asymptomatic carrier

A healthy person who is infected by a pathogen and showing no symptoms of disease. People can become infected with a pathogen and experience no symptoms. That person then can transmit, or “carry” the pathogen to another person, infecting them. That person may then become sick, even if the original carrier does not.

Case fatality rate (CFR)

An estimate of the risk of mortality from a contagious disease. The CFR is calculated by dividing the number of deaths caused by a disease by the number of cases of that disease in a given time period. The CFR is time and location-dependent, and many factors can influence the rate, like age and demographic characteristics, among others. For COVID-19, CFR estimates vary; in China, early CFR estimates by province have ranged from <1% to 5.8%. In May, a United Kingdom research team said the the average global CFR was estimated at about 1.4%.

Droplet transmission/spread

A form of contagious disease spread that involves the spray of saliva or respiratory droplets, expelled when an infected person breathes, coughs, sneezes, talks or sings. If someone is nearby, they could breathe in these droplets and become infected. A person could also touch an item where the infected person’s droplets fell and then become infected when they touch their eyes or mouth. Most droplets are large enough that once they are expelled, they drop to the ground, within the range of about six feet. This is why public health officials ask people to keep six feet apart from others in public and to wash their hands.

Infection-to-fatality rate (IFR)

An epidemiology term that quantifies the chances that a person who contracts an infection from a pathogen, will die from it. Not everyone who is infected by a pathogen will show symptoms of a disease. Not everyone who is infected by a pathogen will die. Knowing the IFR helps scientists determine the danger of a particular pathogen and develop countermeasures to prevent its transmission. With novel pathogens, like with the virus that causes COVID-19, this rate is initially difficult to determine because people who aren’t sick aren’t being tested for infection. As time goes on, and more people are tested for the virus, epidemiologists can determine a more accurate figure for the mortality rate of a pathogen.

Infectious dose

Infectious diseases spread when a healthy person comes in contact with a pathogen expelled by someone sick, such as through a cough, sneeze, sexual activity, or contact with fecal material. The amount of that pathogen necessary to make that healthy person sick is the infectious dose. With SARS-CoV-2, the virus that causes COVID-19, the infectious dose is unknown, but because no one has immunity to it, researchers suspect the dose is low.

Isolation

This policy involves separating people known or suspected to be infected with a contagious disease from those who are not sick to prevent them from transmitting disease to others. The definition of  “suspected” is based on whether the person is showing symptoms of a contagious disease or whether they met certain laboratory criteria demonstrating they have likely been infected.

Multisystem inflammatory syndrome (MIS-C)

This is a rare and potentially life-threatening inflammatory response associated with COVID-19, the respiratory disease caused by the SARS-Cov-2 virus. The syndrome affects mainly children, and emerges weeks after they have contracted the virus. By the end of May 2020, around 300 children had developed MIS in North America and Europe.

Presumptive positive

This is a term used by the Centers for Disease Control and Prevention related to the diagnostic testing process for a disease. A presumptive positive result is when a patient has tested positive by a state or local public health laboratory to infection by a pathogen, but has yet to have been confirmed by the CDC. Public health laboratories are a network of specialized governmental health laboratories that operate at the state and local level across the country. Every state and the District of Columbia, has a public health lab and many states have local public labs in metropolitan areas and smaller communities. They are among the keys to public health surveillance and work closely with the CDC and other federal agencies, as well as international health agencies. 

Positivity rate

The percent of COVID-19 tests with positive results over a seven to 14-day period. As part of public health measures aimed at reducing spread of COVID-19, the World Health Organization recommends that communities aim for a positivity rate of less than 5 percent for at least two weeks . A threshold of 5 percent or more is as an indication that the spread of COVID-19 is not under control in the community.

Quarantine

Involves health authority separating and restricting the movement of people who have potentially been exposed to a contagious disease, until it can be determined whether they have become sick or no longer pose a risk to others. For example, those suspected of exposure to Ebola were quarantined up to 21 days. Quarantines may take place in the home, or other locations determined by health authorities. If a person shows no symptoms of the disease within the time when a person is considered contagious, they are considered disease-free and released from quarantine.

Serologic test

A blood test to detect the presence of antibodies against a microorganism. Antibodies are proteins that the body creates when it detects a foreign presence, such as bacteria or virus.

A serologic test can determine whether a person has been exposed to a particular microorganism, and the potential that they are immune to a disease.

Symptomatic case-fatality rate (sCFR)

An epidemiology term that quantifies the risk that a person who is infected with a pathogen, and showing signs of illness, will die. Knowing this figure helps scientists determine the danger of a particular pathogen. With a fast moving and novel pathogen, like the virus that causes COVID-19, this figure may be a moving target, because only those with illness are tested. As time goes on, and more people are tested for the virus, epidemiologists can determine a more accurate figure for the mortality rate of a pathogen. That is why journalists may want to caveat the mortality rate figures early in an outbreak with something to the effect of: "a mortality rate, based on the information that scientists have."

Viral load

This is the measure of virus particles. Generally, it refers to the amount of virus present in the body, once a person has become infected and the virus has replicated in the person’s cells. For most people, the higher the viral load, the worse the symptoms and outcomes.

Zoonotic

A zoonotic disease refers to a pathogen that has been living within an animal, and then, for an environmental or genetic reason, jumps into the human population where it can cause disease. Two of the best known zoonotic diseases are influenza and the plague. The flu virus lives in the guts of water fowl. The flu can spread to humans through a genetic shift that causes people to become ill. The plague is caused by the bacteria, Yersinia pestis. It can live inside fleas, which then bite humans and cause illness. In 1346, rats carrying fleas with Yersinia pestis, traveled through trade routes in western Europe, causing a pandemic known as the Black Death. Around 60% to 75% of all new diseases that affect humans are zoonotic in origin.