50 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.