Viruses: What are they and what do they do?

Viruses are microscopic organisms that exist almost everywhere on earth. They can infect animals, plants, fungi, and even bacteria.

Sometimes a virus can cause a disease so deadly that it is fatal. Other viral infections trigger no noticeable reaction.

A virus may also have one effect on one type of organism, but a different effect on another. This explains how a virus that affects a cat may not affect a dog.

Viruses vary in complexity. They consist of genetic material, RNA or DNA, surrounded by a coat of protein, lipid (fat), or glycoprotein. Viruses cannot replicate without a host, so they are classified as parasitic.

They are considered the most abundant biological entity on the planet.

Fast facts on viruses

Here are some key points about viruses. More detail is in the main article.

Viruses are living organisms that cannot replicate without a host cell.

They are considered the most abundant biological entity on the planet.

Diseases caused by viruses include rabies, herpes, and Ebola.

There is no cure for a virus, but vaccination can prevent them from spreading.

What are viruses?

Effects of viruses can range from life-threatening to virtually symptomless.

Almost every ecosystem on Earth contains viruses.

Before entering a cell, viruses exist in a form known as virions.

During this phase, they are roughly one-hundredth the size of a bacterium and consist of two or three distinct parts:

genetic material, either DNA or RNA

a protein coat, or capsid, which protects the genetic information

a lipid envelope is sometimes present around the protein coat when the virus is outside of the cell

Viruses do not contain a ribosome, so they cannot make proteins. This makes them totally dependent on their host. They are the only type of microorganism that cannot reproduce without a host cell.

After contacting a host cell, a virus will insert genetic material into the host and take over that host’s functions.

After infecting the cell, the virus continues to reproduce, but it produces more viral protein and genetic material instead of the usual cellular products.

It is this process that earns viruses the classification of parasite.

Viruses have different shapes and sizes, and they can be categorized by their shapes.

These may be:

Helical: The tobacco mosaic virus has a helix shape.

Icosahedral, near-spherical viruses: Most animal viruses are like this.

Envelope: Some viruses cover themselves with a modified section of cell membrane, creating a protective lipid envelope. These include the influenza virus and HIV.

Other shapes are possible, including nonstandard shapes that combine both helical and icosahedral forms.

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Viruses do not leave fossil remains, so they are difficult to trace through time. Molecular techniques are used to compare the DNA and RNA of viruses and find out more about where they come from.

Three competing theories try to explain the origin of viruses.

Regressive, or reduction hypothesis: Viruses started as independent organisms that became parasites. Over time, they shed genes that did not help them parasitize, and they became entirely dependent on the cells they inhabit.

Progressive, or escape hypothesis: Viruses evolved from sections of DNA or RNA that “escaped” from the genes of larger organisms. In this way, they gained the ability to become independent and move between cells.

Virus-first hypothesis: Viruses evolved from complex molecules of nucleic acid and proteins either before or at the same time as the first cells appeared on Earth, billions of years ago


A virus exists only to reproduce. When it reproduces, its offspring spread to new cells and new hosts.

The makeup of a virus affects its ability to spread.

Viruses may transmit from person to person, and from mother to child during pregnancy or delivery.

They can spread through:


exchanges of saliva, coughing, or sneezing

sexual contact

contaminated food or water

insects that carry them from one person to another

Some viruses can live on an object for some time, so if a person touches an item with the virus on their hands, the next person can pick up that virus by touching the same object. The object is known as a fomite.

As the virus replicates in the body, it starts to affect the host. After a period known as the incubation period, symptoms may start to show.

What happens if viruses change?

When a virus spreads, it can pick up some of its host’s DNA and take it to another cell or organism.

If the virus enters the host’s DNA, it can affect the wider genome by moving around a chromosome or to a new chromosome.

This can have long-term effects on a person. In humans, it may explain the development of hemophilia and muscular dystrophy.

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