Virus Glossary
A plain-language explainer of every virus in our risk comparison — what it is, how it spreads, and who it tends to affect. One sentence each, no jargon.
Looking for what kills each of them? See the disinfectant guide by virus type. For the initialisms and jargon that appear in outbreak coverage, jump to terms and abbreviations below.
The 15 Viruses
These definitions are short, plain-language summaries for orientation — they are educational, not medical advice. Case counts and severity figures for each virus, with their CDC / WHO / NIH sources, are on the Virus Risk Perspective comparison.
- Avian influenza A(H5N1)
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A type of bird flu (influenza A H5N1) that mainly infects birds and some mammals; human cases are rare and usually linked to close contact with infected animals, but can be severe.
- COVID-19 (SARS-CoV-2)
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A respiratory illness caused by the SARS-CoV-2 coronavirus and spread mainly through the air; most cases are mild, but it can cause severe pneumonia, especially in older adults and people with underlying conditions.
- Dengue
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A mosquito-borne viral infection common in tropical and subtropical regions that causes high fever and body aches; a minority of cases progress to severe, potentially fatal dengue.
- Ebola virus disease
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A rare but often deadly viral hemorrhagic fever spread through contact with the body fluids of infected people or animals; outbreaks occur mainly in Central and West Africa.
- Hantavirus (hantavirus pulmonary syndrome, US)
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A group of viruses spread chiefly by breathing in air contaminated with infected rodent droppings, urine, or saliva; in the Americas it can cause hantavirus pulmonary syndrome, a severe and sometimes fatal lung disease.
- Marburg virus disease
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A rare and severe viral hemorrhagic fever related to Ebola, spread through contact with infected body fluids; outbreaks are sporadic and often highly fatal.
- Measles
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A highly contagious virus that spreads through the air and causes fever and a distinctive rash; it can lead to serious complications such as pneumonia and brain swelling, and is preventable by vaccine.
- Mpox (monkeypox)
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A viral disease (formerly called monkeypox) that causes a painful or itchy rash along with flu-like symptoms, spread mainly through close skin-to-skin contact.
- Norovirus
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A highly contagious virus that causes vomiting and diarrhea (gastroenteritis), spread through contaminated food, water, surfaces, or close contact; it is the most common cause of foodborne illness in the United States.
- Powassan virus
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A rare tick-borne virus found in North America that can cause encephalitis (brain inflammation); most infections cause no symptoms, but severe cases can be life-threatening.
- Rabies (US)
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A virus spread through the bite of an infected animal that attacks the nervous system; it is almost always fatal once symptoms begin, but preventable with prompt post-exposure treatment.
- Respiratory syncytial virus (RSV)
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Respiratory syncytial virus, a common seasonal respiratory virus that usually causes mild, cold-like symptoms but can be serious for infants and older adults.
- Seasonal influenza
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The seasonal flu, a contagious respiratory illness caused by influenza viruses that circulate each winter; it ranges from mild to severe and is preventable by annual vaccination.
- West Nile virus
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The leading mosquito-borne virus in the continental United States; most infected people have no symptoms, but a small share develop serious neurological illness.
- Zika virus
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A mosquito-borne virus that usually causes mild or no symptoms in adults but can cause serious birth defects when a woman is infected during pregnancy.
Terms & Abbreviations
28 pieces of jargon you meet in outbreak coverage, in plain language — with what each one actually tells you, and where on this network it is used. Most are initialisms nobody expands on first use.
The two most consequential ones are CFR and IFR. They are different denominators for the same question, they differ by an order of magnitude for the same disease, and they are swapped constantly. If a fatality figure does not say which it is, it is not telling you much.
Measuring how dangerous a virus is
- CFR case fatality rate
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Deaths divided by diagnosed cases. It runs high for diseases only counted once someone is sick enough to be tested — Ebola, hantavirus, rabies, Marburg.
The headline number in most outbreak reporting · see it in context →
- IFR infection fatality rate
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Deaths divided by all infections, including the mild and undiagnosed ones. Always lower than CFR for the same disease, and much harder to measure — it needs seroprevalence surveys, not case counts.
COVID-19 debates, where CFR and IFR were routinely confused
- R₀ basic reproduction number, said "R-nought"
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The average number of people one infected person infects in a population with no immunity and no controls. A property of virus and setting, not of the virus alone — which is why published values for the same disease differ.
Outbreak modelling coverage · see it in context →
- Rt effective reproduction number
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The same measure under real conditions — with immunity, behaviour change and controls in play. Above 1 an outbreak grows; below 1 it shrinks.
Situation reports during an active outbreak
- Attack rate
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The share of an exposed group that falls ill. Used for contained settings — a cruise ship, a wedding, a care home — where the denominator is actually known.
Cruise-ship norovirus reporting · see it in context →
- Seroprevalence
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The share of a population carrying antibodies to a virus, from blood testing. The usual way to estimate how many infections were never diagnosed.
Disease names you will meet as initials
- PASC post-acute sequelae of SARS-CoV-2 infection
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The formal clinical name for Long COVID — the symptoms that persist or appear weeks to months after the acute infection clears. Used in the research literature and by the NIH RECOVER initiative where the press says "Long COVID".
Research papers and clinical guidance on Long COVID · see it in context →
- EVD Ebola virus disease
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Disease caused by any of the human-infecting ebolavirus species. Some sources now name the species — BVD for Bundibugyo virus disease — because the available vaccines and antibodies are species-specific.
WHO and CDC outbreak reporting · see it in context →
- HPS / HCPS hantavirus pulmonary syndrome / hantavirus cardiopulmonary syndrome
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The severe lung-and-heart form of hantavirus infection seen in the Americas. The two initialisms mean the same illness; "cardiopulmonary" is more common in South American sources.
US and South American hantavirus surveillance · see it in context →
- HFRS haemorrhagic fever with renal syndrome
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The kidney form of hantavirus infection, seen in Europe and Asia. A different disease from HPS despite the shared virus genus — and far less lethal, which is why "the hantavirus fatality rate" is meaningless without naming a continent.
ECDC and Asian surveillance reports · see it in context →
How the virus gets to you
- Enveloped / non-enveloped
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Whether a virus carries a fatty outer membrane. Enveloped viruses (coronaviruses, ebolaviruses, hantaviruses, influenza) are broken apart by alcohol and detergents; non-enveloped ones (norovirus) are not. This single distinction explains most disinfectant confusion.
Cleaning and disinfection guidance · see it in context →
- Zoonosis / spillover
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A zoonosis is an infection that moves from animals to people; spillover is the moment it happens. Every virus on this site is zoonotic in origin.
- Incubation period
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Time from infection to first symptoms. Distinct from the latent period, the time from infection until a person becomes infectious — which can be shorter, and is why some diseases spread before anyone feels ill.
Quarantine and monitoring rules · see it in context →
- Serial interval
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The gap between symptom onset in one person and in the person they infect. Used to estimate how fast an outbreak can grow.
- Fomite
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An inanimate object that carries infection — a door handle, a bed rail, a shared toy.
Norovirus and surface-survival discussion · see it in context →
Who publishes what
- MMWR Morbidity and Mortality Weekly Report
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CDC's weekly journal. Where US outbreak investigations are first published, often as short “Notes from the Field” pieces.
- DON Disease Outbreak News
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WHO's running situation reports on an active outbreak, numbered sequentially. The primary source for case counts during an emergency.
- NNDSS National Notifiable Diseases Surveillance System
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The US weekly case-count system. Its figures are provisional and routinely revised upward for months, which is why a year-end number is not a final number.
US annual case counts · see it in context →
- NORS National Outbreak Reporting System
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CDC’s database of reported enteric-disease outbreaks, including setting — restaurant, school, care home, cruise.
- EID Emerging Infectious Diseases
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CDC's peer-reviewed monthly journal, and a common source for outbreak-summary papers.
Emergency and regulatory language
- PHEIC Public Health Emergency of International Concern, said "fake"
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WHO's highest formal alarm, declared by the Director-General on an Emergency Committee's advice. It unlocks coordination and funding; it is not a statement about how deadly a disease is.
Declared for the 2026 Bundibugyo outbreak · see it in context →
- EUA Emergency Use Authorization
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FDA permission to use an unapproved product, or an approved one in a new way, during a declared emergency. Not the same as approval — molnupiravir has an EUA and has never been FDA-approved.
COVID antivirals and vaccines · see it in context →
- PUI person under investigation
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Someone being evaluated for a disease before testing confirms or excludes it. PUIs appear and vanish from case counts, which is one reason early outbreak figures move.
- VOC / VOI variant of concern / variant of interest
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WHO classifications for virus variants by evidence of changed transmissibility, severity or immune escape. A VOC is the more serious label.
COVID variant tracking · see it in context →
- SAGE / ACIP WHO's Strategic Advisory Group of Experts on Immunization / CDC's Advisory Committee on Immunization Practices
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The expert committees that recommend how vaccines are used — globally and in the US respectively. Their recommendations are separate from regulatory approval.
Protective equipment
- PPE personal protective equipment
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Gloves, gowns, eye protection and respiratory protection, used together in a specified order.
Ebola and rodent-cleanup protocols · see it in context →
- N95 / PAPR
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An N95 is a filtering facepiece respirator that captures at least 95% of airborne particles and needs a proper seal. A PAPR is a powered air-purifying respirator with a HEPA filter — used where a tight seal is impractical or exposure is prolonged.
CDC's heavy-infestation rodent cleanup list · see it in context →
- Contact / droplet / airborne precautions
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The three transmission-based precaution tiers in CDC's infection-control framework, layered on top of standard precautions. Which tier applies determines the room, the mask and the procedure.
These are plain-language explanations for orientation, not medical advice and not formal clinical definitions. Where a term has a precise regulatory or clinical meaning — EUA, PHEIC, the precaution tiers — the issuing body's own wording governs, and it is linked from the page that uses the term.