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What's a spillover? A spillback? Here are definitions for the vocab of a pandemic

Olivia Taussig-Rees for NPR

Don't feel as if you're out of the loop if you're not up on the terminology of spillover viruses.

In fact, one of the doctors we interviewed for this series on spillovers asked, "What is your definition of spillover?" She wasn't exactly sure herself — and her field is infectious diseases.

So, here's a glossary of terms that you will see during our series, starting of course with "spillover."

These terms are broadly organized with related words and concepts grouped together rather than alphabetically. We're starting with some of the big stuff — the most important terms to know. Farther down the list, you'll find terms that are a little bit more specialized but still are helpful in understanding the world of spillover viruses.

So let's begin.

Spillover: The transmission of a pathogen from an animal to a human. In most cases, spillover does not cause the human to get sick or transmit the pathogen to other humans.

Pathogen: An infectious agent with the potential to cause disease. Pathogens include viruses, bacteria, fungi, parasites and prions.

Zoonosis: A pathogen that has spread from animals to humans. Not all pathogens are zoonoses. Zoonoses can be spread through direct contact with an infected animal or through contaminated food or water. Zoonoses can come from both domesticated and wild animals.

Spillback (reverse spillover): The transmission of a pathogen from humans to animals. Much like spillover from animals to humans, during spillback the infected animal may or may not get sick. For example, COVID-19 has been transmitted from humans to deer and mink, among other animals.

Exposure: Contact with a pathogen. Exposure does not always result in an infection.

Shedding: The release of a virus or other pathogen from an infected person into the environment. Those pathogens that have been shed into the environment can often be infectious, and this is how the pathogen gets transmitted from one person to another.

Outbreak: Rapid spread of an infection among a community.

Epidemic: Unexpected rapid or extensive spread of a pathogen that is contained to a specific area or region. Epidemics are larger than a typical outbreak and typically prompt an emergency response from global health organizations.

Pandemic: Unexpected rapid or extensive spread of a pathogen that is no longer contained to a specific region and instead has spread across several countries or across the globe.

Endemic: An endemic pathogen maintains a consistent presence within a population or region.

Host: A human or animal that is a carrier for a pathogen.

Vector: An organism that transmits a pathogen to other organisms, typically through direct contact. For example, Anopheles mosquitoes are vectors for malaria, which is transmitted through bites.

Reservoir: The place where a pathogen normally lives and reproduces. For example, rodents are reservoirs for plague bacteria, which can then be spread to humans and other animals through a mosquito vector.

Pandemic potential: The potential of a virus or other pathogen to cause a pandemic. Scientists have found that certain traits, such as a virus having genetic material made of RNA, make that pathogen more likely to cause a major outbreak of disease.

Patient zero: The person with the first known or suspected case of infection by a pathogen that goes on to cause an epidemic or pandemic.

Virus: A nonliving infectious agent that requires a host to reproduce. Viruses do not have a cellular structure and their genetic material can be based from DNA or RNA. About 270 viruses are known to infect people and cause a variety of diseases, including COVID-19, HIV and Ebola. However, scientists estimate there are hundreds of thousands of unknown viruses on Earth with the potential to infect mammals.

Bacteria: Bacteria are small single-celled organisms found nearly everywhere on Earth. Most of them do not affect humans at all. However, some bacteria can cause disease and other bacteria provide benefits to us humans.

Fungi: Fungi are a group of multicellular living organisms that include mold, yeast and mushrooms. Some microscopic forms of fungi are infectious and can cause disease in humans.

Parasite: Parasites are complex living organisms that can cause disease. They are different from viruses, bacteria or fungi, but can have similar effects on a host. For example, malaria is caused by the parasite Plasmodium.

Prion: An infectious protein that can replicate and cause disease. Prion-based diseases are rare but almost always fatal. Prions affect the brain and typically take a long time to develop. One example is mad cow disease.

Antibodies: Antibodies are proteins produced by the immune system that fight off infections. Antibodies bind to foreign substances in the body called antigens and destroy them.

Antigens: An antigen is any foreign substance or protein that induces an immune response in the body. Antigens can be viruses, bacteria, fungi or any other substance that enters the body.

Mortality: Mortality is often used in a medical context to express the number of deaths an infection causes. It is often expressed as a mortality rate: the number of known deaths in a population. For example, in the United States, the mortality rate for the flu is about 16 people per 100,000.

Risk: Risk is often used to describe the chances of being affected by a disease or other outcome. It is typically expressed as 1 out of a larger number. For example, if you live in the U.S., your risk of being killed in a car accident last year was 1 in 7,500. Risk is not identical for all groups of people and is dependent upon factors such as medical conditions or behavior. For example, if you don't drive a car, your risk of being killed in a car crash is much lower.

Comorbidity: Comorbidity is a term used to describe when a patient has two or more medical conditions at the same time. In some, but not all circumstances, those medical conditions can interact with each other, resulting in more severe disease in the patient.

Mutation: A change in an organism's genetic code. Most mutations have no discernible effect. Many mutations have a negative effect on the organism and fewer mutations are positive for the organism.

Genomic sequencing: A laboratory method of reading the genetic material of an organism. Sequencing the genetic material of an organism or pathogen helps scientists uncover the function of genes, find mutations, and answer questions about the sample's evolutionary origin. Either DNA or RNA can be sequenced to answer these questions depending on the organism's genetic makeup and the specific question the scientist is hoping to answer.

Genomic surveillance: Tracking the spread and evolution of a pathogen through genomic sequencing of samples from infected individuals. For example, genomic surveillance has allowed scientists to detect new variants of COVID-19 and track their spread across the globe. Genomic surveillance can be performed in humans, animals, and even environmental samples such as wastewater from sewage treatment plants.

Copyright 2023 NPR. To see more, visit https://www.npr.org.

Corrected: February 6, 2023 at 11:00 PM CST
In defining mortality rate, we incorrectly stated that it is calculated by dividing the number of deaths by the number of infections over a specified period of time. That calculation yields the case fatality rate. Mortality rate is the number of deaths in a population over a period of time — for instance, the number of deaths per 1,000 people each year.
Max Barnhart
Max Barnhart is the 2022 AAAS Mass Media Science and Engineering Fellow at NPR. He is a 5th year Ph.D. candidate and science journalist studying the evolution of heat stress resistance in sunflowers at the University of Georgia.
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