100 Fun Facts About Viruses

Where can a virus replicate?

Viruses have no metabolism of their own, so they hijack a host cell's machinery to build copies.

What is the study of viruses called?

It is a subspeciality of microbiology, dating from Beijerinck's work in the 1890s.

What is a single virus particle outside a cell called?

The term dates from 1959 and describes the released particle capable of infecting another cell.

What is the protein coat that surrounds a virus's genetic material called?

It is built from repeating protein subunits called capsomeres; some viruses add an outer envelope of lipids.

Which plant pathogen, found in filtered sap in the 1890s, was the first of its kind ever described?

Dmitri Ivanovsky showed that sap from diseased plants stayed infectious after passing through a filter fine enough to trap bacteria.

Which Dutch scientist coined the modern use of 'virus' in 1898 after studying diseased tobacco plants?

Ivanovsky had run the filter experiment six years earlier, but the 1898 work argued the agent could reproduce only inside living cells.

The word "virus" comes from a Latin word meaning what?

It shares a root with the Sanskrit and Ancient Greek words for poison; the sense of "agent causing infectious disease" is first recorded in 1728.

Why do many biologists hesitate to call viruses alive?

They have genes, evolve by natural selection and self-assemble, which is why they are called "organisms at the edge of life".

What genetic material can a virus carry?

Most viruses have RNA genomes; retroviruses convert theirs into DNA at one stage of the cycle.

Roughly what size are most studied spherical viruses?

More than a thousand of them would fit inside a single E. coli cell.

Which instrument, invented in 1931, finally allowed the complex structures of viruses to be seen?

Wendell Stanley had already shown that viruses were particles rather than a fluid.

How few norovirus particles can be enough to infect a person?

Norovirus and rotavirus spread by the faecal-oral route, in food, water or hand-to-mouth contact.

What term describes which cell types a given virus is able to infect?

Rabies can take hold in many mammal species, while other viruses are restricted to a single one.

What are viruses that infect bacteria called?

At up to 250 million per millilitre, they are the most abundant biological entity in seawater.

Roughly how many viruses are there in a teaspoon of seawater?

Most of them attack bacteria and cyanobacteria, and they kill an estimated 20% of ocean microbial biomass every day.

What is the "viral shunt" in ocean ecosystems?

Organic molecules from the dead cells feed fresh bacterial and algal growth, and boost nitrogen cycling.

How many viruses were reported in 2018 to fall from the atmosphere onto every square metre of Earth daily?

They circulate above the weather but below usual airline altitude, spreading around the planet.

Which pond ciliate was reported in 2022 to eat viruses, the first observed case of 'virovory'?

It multiplied on a diet of chlorovirus once all other microbial food was removed from the water.

Which giant virus, with a 400 nm particle, helped overturn the idea that all viruses are tiny?

Megavirus chilensis, found off Chile in 2011, is big enough to be seen with a basic optical microscope.

Which giant virus genus, found in 2013, has genomes about twice the size of the previous record?

Their two-megabase genomes code for roughly 2,500 proteins; all giant viruses have double-stranded DNA.

What are viroids, the subviral agents that attack plants?

They are significant plant pathogens and are often called subviral agents.

What are viruses that can only replicate in a cell already infected by another virus called?

The sputnik virophage, for instance, can only replicate in an amoeba that is already infected by a giant virus.

Which biologist devised the classification system that sorts viruses by how they make messenger RNA?

His scheme is used alongside the ICTV's taxonomic classification.

How many groups does the Baltimore classification place viruses into?

They range from double-stranded DNA viruses to reverse-transcribing viruses such as retroviruses and hepadnaviruses.

Which enzyme lets retroviruses turn their RNA genome into DNA?

Drugs such as zidovudine and lamivudine work by inhibiting it.

Why do RNA viruses generally have smaller genomes than DNA viruses?

Beyond a certain size, copying mistakes would render the particle useless, so many RNA viruses split their genome into segments.

Which viruses have the smallest known genomes, coding for just two proteins?

Their genome is about two kilobases, roughly a thousand times smaller than the largest known viral genomes.

What is the first stage of a virus's replication cycle?

HIV's gp120 protein binds the CD4 molecule, which is why it targets a narrow set of human leucocytes.

What is the bursting process that kills a host cell as new viruses escape?

Enveloped viruses such as HIV usually leave more gently, by budding through the cell membrane.

What is a viral genome called once it has integrated into a bacterium's chromosome?

In other hosts the integrated genome is called a provirus; it is copied every time the cell divides.

What term describes a virus lying dormant in a cell for months or years, as herpes viruses do?

Epstein-Barr virus and varicella zoster, which causes chickenpox and shingles, behave the same way.

Which antibody in the blood indicates a recent, acute viral infection?

IgG, produced indefinitely, is the one measured when testing for immunity.

Which protein does the body produce to stop viruses reproducing, killing infected cells?

It is part of the innate immune response, alongside RNA interference.

How does HIV evade the immune system?

The trick is called escape mutation: the viral epitopes change faster than the immune system can track them.

Roughly how many people did the 1918 influenza pandemic kill, by the highest estimates?

Unusually, its victims were often healthy young adults rather than the very young or old.

Ebola and Marburg viruses belong to which family of filament-shaped viruses?

Marburg virus was first identified in 1967 and Ebola in 1976.

In which Chinese city did SARS-CoV-2 emerge in late 2019?

The virus is thought to have originated in bats, and triggered the COVID-19 pandemic in 2020.

Which cancer virus, discovered in 2008, causes a rare skin cancer?

It is behind most cases of Merkel cell carcinoma and was identified in 2008.

Which virus is an established cause of cancers of the cervix, anus and penis?

Hepatitis B and C, by contrast, can lead to liver cancer through chronic infection.

Which viral disease of humans has been eradicated by vaccination?

Only two labs are authorised to keep stocks of the virus, in Russia and the United States.

How many centres worldwide may keep WHO-authorised stocks of the eradicated pox-scarring virus?

They are in Russia and the United States; routine vaccination stopped because of side effects, leaving most people unprotected.

What are live vaccines made from viruses that have been weakened called?

They can be risky for immunocompromised people, which is why subunit vaccines using only capsid proteins exist.

Nucleoside analogues are a major class of antiviral drug. How do such drugs work?

Aciclovir, used against herpes simplex, is one of the oldest and most prescribed examples.

What term describes a pathogen engineered to destroy cancer cells?

T-VEC, a modified herpes simplex virus, was approved for melanoma in late 2015.

What defence system do bacteria use to store fragments of viral genomes they have met before?

Most archaea have the same adaptive defence against their own viruses.

Which aphid-borne pathogen of a staple tuber crop can cut yields by as much as 80 per cent?

It also attacks tomatoes and peppers, and became a serious problem in seed potato crops in the 1980s.

In what year was the first synthetic virus created from scratch?

What is synthesised is the genome, not the particle; the technique means a virus can never truly be extinct while its sequence is known.

Which Russian scientist showed in 1892 that filtered tobacco sap stayed infectious?

His filter experiment was the first evidence that something smaller than bacteria could cause disease, six years before Beijerinck named it a virus.

In what year was the International Committee on Taxonomy of Viruses (ICTV) formed?

By 2024 the ICTV had defined 7 realms, 368 families and more than 16,000 virus species, using a 15-rank system adopted from 2018.

Which disease's live-attenuated 17D strain is called probably the safest shot ever generated?

Subunit vaccines such as the hepatitis B shot use only capsid proteins and are safe for immunocompromised patients, unlike live vaccines.

In the 17th-century Dutch tulip mania, a virus gave the most sought-after bulbs which prized feature?

Flame-streaked petals were a symptom of tulip breaking virus, which nobody then knew existed. Celebrated broken varieties such as Semper Augustus and the Viceroy no longer exist.

Which US president, diagnosed with polio in 1921, founded the charity that became the March of Dimes?

Franklin Roosevelt launched it in 1938 as the National Foundation for Infantile Paralysis. Entertainer Eddie Cantor coined the catchier name as a pun on the newsreel series The March of Time.

For decades, most flu vaccines have been made by growing the virus inside what?

Fertilised chicken eggs were already growing the virus for the first approved flu vaccines of the 1940s. Flu vaccines grown in cell culture were not approved in the United States until 2012.

Ebola virus, first identified in 1976, is named after what kind of feature near the outbreak site?

A river, the Ebola, runs near the village of Yambuku in what was then Zaire, scene of one of two simultaneous 1976 outbreaks. The other struck Nzara, in what is now South Sudan.

MERS, a coronavirus first found in Saudi Arabia in 2012, usually spreads to people from which animals?

Camels pass the virus on through direct contact or respiratory droplets, earning MERS the nickname camel flu. Most cases have occurred in the Arabian Peninsula.

Edward Jenner's first vaccine used a sample from Sarah Nelmes, who had caught which mild disease?

Cowpox gave vaccination its name, from vacca, the Latin for cow. Jenner took his sample from Nelmes, a milkmaid, rather than straight from a cow's udder.

Which fatal viral disease was long known as hydrophobia because sufferers panic when offered a drink?

Rabies triggers painful throat spasms on swallowing, hence the old name. Where dogs commonly carry the disease, their bites cause more than 99% of human cases.

The name coronavirus, coined by June Almeida and David Tyrrell, comes from a Latin word meaning what?

A crown, or wreath, is what the fringe of spikes on each particle suggested to its namers, who likened the look to the solar corona. The word first appeared in print in 1968, in the journal Nature.

The pandemic strain of HIV, known as group M, jumped to humans from which primate in Central Africa?

Chimpanzees in Cameroon carry SIVcpz, the direct ancestor of pandemic HIV. The human strain began spreading in Léopoldville, now Kinshasa, in the 1920s.

Mpox got its original name because the virus was first detected in 1958 in which laboratory animals?

Monkeys kept for research fell ill with a pox-like disease, yet there is little evidence that any monkey species is a natural reservoir. The WHO announced the name mpox in November 2022.

Which American virologist's polio vaccine was hailed as a success in April 1955 and was never patented?

Jonas Salk chose a killed-virus vaccine over the live weakened strains that Albert Sabin was developing. He declined to patent it so that it could be distributed as widely as possible.

Which machine, enclosing most of the body, kept polio patients with paralysed diaphragms breathing?

Iron lung machines filled hospital wards during the polio outbreaks of the 1940s and 1950s. The device was first used to treat coal gas poisoning.

Which contagious viral disease produces white Koplik spots inside the mouth before its rash appears?

Measles can also wipe out immune memory by killing the cells that make antibodies, an effect called immune amnesia. That leaves survivors open to infections they had already beaten.

Which viral infection causes painful swelling of the parotid salivary glands at the sides of the face?

Mumps vaccine uses the Jeryl Lynn strain, which Maurice Hilleman took from his own five-year-old daughter. His first version was licensed in 1967.

Which usually mild rash illness, first told apart by German doctors in 1814, can cause severe birth defects?

Rubella caught early in pregnancy can cause miscarriage or congenital rubella syndrome in the baby. After the 20th week of pregnancy, problems are rare.

Maurice Hilleman, developer of over 40 vaccines, merged three into which childhood shot licensed in 1971?

MMR was licensed in the United States by Merck, the firm where Hilleman developed most of his vaccines. One injection protects against three viral diseases.

Which group of viruses, with at least 165 recognised types, is the predominant cause of the common cold?

Rhinoviruses take their name from the Greek for nose, their main way into the body. They cause at least half of all colds, and by some estimates as many as 80%.

Hepatitis, which five different viruses lettered A to E can cause, is an inflammation of which organ?

The liver can be scarred by chronic hepatitis, a condition called cirrhosis, and may eventually fail or turn cancerous. Alcohol, drugs and autoimmune disease can inflame it too.

Why do antibiotics such as penicillin do nothing to cure a cold or the flu?

They only act on bacteria, so the viruses behind colds and flu are untouched by them. Antibiotics are no use against fungi either.

What term describes an infectious disease, such as Ebola, that passes from animals to humans?

Zoonosis covers a large share of human infections: of 1,415 pathogens known to infect people, 61% were found to be zoonotic. Salmonellosis is another example.

HIV leads to AIDS by steadily depleting which class of white blood cell, marked by the CD4 protein?

Helper T cells are among the vital immune cells that HIV infects. Once their numbers fall below a critical level, the body becomes ever more prone to opportunistic infections.

Which drug, better known by the initials AZT, was approved in 1987 as the first treatment for HIV?

Zidovudine was first described in 1964, more than two decades before it found its use. Its other name, azidothymidine, supplies the initials.

Which mosquito-borne disease did physician Benjamin Rush call break-bone fever for its joint pains?

Dengue picked up the nickname in a 1789 report on an epidemic that had struck Philadelphia in 1780. Rush, its author, was also a Founding Father of the United States.

Which daytime-biting genus of mosquito, recognisable by white markings on its legs, is the main carrier of Zika virus?

Aedes aegypti, the chief culprit, also carries a lyre-shaped mark on its thorax. Males live off fruit, and only the female bites for blood.

Which virus, given its name in 1974, is the leading cause of severe diarrhoea in infants?

Rotavirus takes its name from rota, the Latin for wheel, a label suggested by Thomas Henry Flewett. Infections have fallen sharply in countries that vaccinate babies against it.

Zika virus is named after a forest in which African country, where it was first isolated in 1947?

Uganda's Ziika Forest, near Lake Victoria, is where scientists isolated the virus from a caged rhesus macaque. Human infection was first reported in 1952.

Marburg virus is named after a city in which European country, where lab workers fell ill in 1967?

Germany's outbreak began at the Behringwerke plant, where staff handled tissue from infected grivets, an African primate. Frankfurt and the Yugoslav capital Belgrade were hit the same year.

Chinese scientists traced the SARS virus from cave-dwelling horseshoe bats to people through which intermediate mammal?

Palm civets were found to carry SARS-like coronaviruses, and more than 10,000 of them were culled in Guangdong Province. The bats at the start of the chain lived in Yunnan.

The first known Nipah virus outbreak, in Malaysia in 1998, spread to people from which farm animals?

Pigs on Malaysian farms fell ill first, and about a million of them were culled to end an outbreak that infected 265 people and killed more than 100.

What term describes the indirect protection that unvaccinated people gain when most of a community is immune?

Herd immunity was first used as a term in 1894 by the American veterinary scientist Daniel Elmer Salmon, who was describing the disease resistance of well-fed hogs.

Hantaviruses are named after the Hantan, a waterway near which soldiers fell ill during which war?

Korean War troops stationed near the Hantan commonly came down with a haemorrhagic fever. The first hantavirus was not isolated until 1978, in South Korea.

Which virus group, named after a small town in New York State, includes the cause of hand, foot and mouth disease?

Coxsackievirus was discovered in 1948–49 by Gilbert Dalldorf, who obtained his first specimens in Coxsackie, a small town on the Hudson. Its group A types infect the skin and mucous membranes.

Which virus, discovered in 1975, causes the childhood rash known as fifth disease or slapped cheek syndrome?

Parvovirus B19 can infect people of all ages, and about two in ten of them show no symptoms at all. The rash it causes is formally called erythema infectiosum.

The 2013–2016 West African Ebola epidemic began with a toddler's death in a village in which country?

Guinea recorded the first cases in December 2013, in the village of Méliandou. The disease then spread to Liberia and Sierra Leone, with minor outbreaks in Nigeria and Mali.

Which mosquito-borne virus, first identified in 1952, has a name from Makonde words meaning 'to become contorted'?

Chikungunya describes the bent posture of people with its chronic symptoms. The name came out of an outbreak on the Makonde Plateau, on the border of Mozambique and what is now Tanzania.

The letter N in the names of influenza A subtypes stands for which enzyme on the surface of the virus?

Neuraminidase cuts newly made virus particles free of the infected cell, while its partner hemagglutinin, the H, lets the virus latch on. The numbers state which version of each protein a subtype carries.

What name is given to the slow build-up of mutations that lets flu viruses evade last season's antibodies?

Antigenic drift changes the parts of the virus that antibodies recognise, one mutation at a time. The more dramatic change that comes when two viruses mix their genetic material is called antigenic shift.

Which influenza A subtype caused both the 1918 pandemic and the 2009 swine flu pandemic?

H1N1 was also behind the Russian flu of 1977, a mild pandemic that mostly struck people under 26. Its strain was almost identical to one isolated in 1950.

Which viral cattle disease was declared eradicated worldwide in June 2011 after a decades-long campaign?

Rinderpest is German for cattle plague, and its last confirmed case was diagnosed in 2001. The virus also struck buffalo, antelope, giraffes, wildebeest and warthogs.

Which viral disease was deliberately released in Australia in 1950 to control feral rabbits?

Myxomatosis was spread using a virus strain taken from the tapeti, a South American rabbit. France and Chile released the virus in the same decade.

Which fruit crop in Hawaii was rescued in 1998 by genetically engineered varieties that resist a ringspot virus?

Papaya production in Hawaii had been halved before the transgenic Rainbow and SunUp varieties arrived. Both were created by Dennis Gonsalves and his team.

A gene left behind by an ancient virus infection makes syncytin-1, a protein essential for forming which organ?

The placenta needs syncytin-1 to fuse cells together as it develops. The gene is the remnant of a retrovirus that worked its way into the primate germ line long ago.

Which immunologist shared the 2023 Nobel Prize in Medicine with Katalin Karikó for work behind mRNA COVID-19 vaccines?

Drew Weissman met Karikó at a university photocopier, where the two commiserated over the lack of funding for RNA research. Their collaboration began there.

What name is given to R0, a measure of how many new cases one case of an infection is expected to cause?

Basic reproduction number is pronounced R nought, and it assumes a population in which everyone is still susceptible. A different figure, R at time t, applies once some people no longer are.

What name is given to an inanimate object, such as a doorknob, that can carry infection to a new host?

A fomite spreads germs best when it is smooth: non-porous surfaces such as door knobs pass on bacteria and viruses more readily than paper money, whose fibres trap them.

Which protein on human cells serves as the main entry receptor for the coronaviruses behind SARS and COVID-19?

ACE2, short for angiotensin-converting enzyme 2, sits in the cell membrane. Another human coronavirus, HCoV-NL63, gets in through the same receptor.

Adeno-associated virus, long thought harmless, became the gold-standard delivery vehicle in which field of medicine?

Gene therapy uses replication-defective versions of the virus to carry genes into cells. In nature, AAV turns up in people already infected with adenovirus, hence its name.

Children recovering from flu or chickenpox are told to avoid which painkiller because of Reye syndrome?

Aspirin is linked to about 90% of childhood cases of Reye syndrome, a rapidly worsening brain disease. Rates fell by more than 90% after the US and UK stopped giving it to children in the 1980s.

Croup, the childhood illness with a barking cough likened to a sea lion's call, is most often caused by what?

Parainfluenza virus, mainly types 1 and 2, is behind about 75% of viral croup. The infection swells the inside of the windpipe, which produces the bark and a hoarse voice.

Bluetongue, a viral disease of sheep and other ruminants, is spread mainly by which tiny biting insects?

Midges of the genus Culicoides carry the virus, so it persists wherever the climate lets them survive the winter. Goats, cattle and antelope can be infected too.