The common cold — runny nose, feeling all stuffed up. You can thank a virus for that. COVID-19 is also caused by a virus. But what exactly is a virus?
1. What is a virus’s body made of?
A virus is a string of genetic material, either DNA (deoxyribonucleic acid) or RNA (ribonucleic acid), that is surrounded by an envelope of protein. The envelope is called a capsid.
Illustration by Amelia Case.
Unlike organisms in the other kingdoms of life, a virus is not made from cells. A virus is much smaller than most cells.
Virologists (biologists who study viruses) do not agree on whether or not a virus can be called an organism. A virus cannot reproduce on its own. It enters a cell and uses the machinery of the cell to make many, many copies of itself. The genetic material in the virus and its capsid take over the cell’s chemical processes for copying genetic material.
The life cycle of a virus. Illustration by Amelia Case.
2. How do viruses eat?
Viruses do not eat. They do not breathe. They do not sleep.
3. Are viruses motile (moving) or sessile (not moving)?
A virus is a piece of genetic material that becomes active when it enters a cell. Otherwise, the virus is just floating around in its environment until it finds a cell in which it can reproduce itself.
Earth is one big virome. A virome is the collection of all the viruses in a particular area. Your nose has a virome. A tree has a virome. Any specific area where you choose to count all the viruses is a virome.
A virome in a park. Click the question marks to explore the viruses. Illustrations by Amelia Case.
Viruses are tiny, and there are a LOT of them. One scientific estimate is that there are as many as 10³¹ viruses on Earth. That is 10 nonillion, or a 1 followed by 31 zeros. There may be more viruses on Earth than stars in the universe.
Viruses are invisible to us without extremely powerful microscopes or by sequencing their genetic codes so we know they are present. Viruses are swirling around us, everywhere, all the time. Most of them do not make us sick or have any noticeable effect on us. But many illnesses like colds, flu, and measles are caused by viruses. Warts are caused by viruses. In our lifetimes, we will all probably feel the effects of viruses, but we are mostly unaware of them.
4. How long have viruses been on Earth?
Viruses have possibly been around for 4 billion years. Earth has been around for 4.5 billion years, so viruses have experienced a very long history. Scientists are still debating whether viruses evolved after or before cells.
Virus fun facts
How many virus species are there?
Illustration by Amelia Case.
A German scientist named Adolf Mayer first described the effects of a virus in 1886. He showed that the sap of a tobacco plant could transmit a disease caused by a tiny germ. Other scientists in the 1890s showed that the tiny germ that Mayer described was a virus.
Since the 1890s, virologists have described more than 11,000 species of viruses. They still have a lot of work to do, though, as there are possibly a billion more viruses on Earth to describe.
Every species of organism on Earth, from every kingdom, may have at least one kind of virus found only in it.
Largest known virus species:
The largest known virus is about one-third the width of a human red blood cell. With so many undiscovered viruses, though, there may be larger ones out there.
Smallest known virus species:
The smallest known virus is about 1/6000th the width of a human hair.
Written by David Brown, all illustrations © Amelia Case
