Viruses Facts

Viruses ['vī-rəs-əz]

Tiny one-celled living organisms, also called microbes, too small to be seen with your eyes.

What Is A Virus?

Respiratory Syncytial Virus

Viruses are in the news. You've certainly heard about COVID-19 and the coronavirus. You may also know about diseases like measles, chicken pox, rabies, or polio, and you may have heard of Ebola, West Nile illness, SARS, or AIDS. You've probably heard of the flu, and you've most likely had a "cold" -- your nose is runny, you sneeze a lot, you have a sore throat, and you might feel achy all over. As it turns out, all of these diseases are caused by viruses.

Electron Microscope
Electron Microscope

A virus is a microbe, too small to be seen with just your eyes. Other types of microbes are bacteria, protozoa, and fungi. Microbes are all around us. They exist on our skin, in water, on desks and doorknobs, and even in food. Microbes are often called "germs," but not all are harmful. Bacteria, for example, make yogurt, pickles, and cheese, and even break down garbage. Learn more about different types of microbes.

Viruses are different from other kinds of microbes. They are much, much smaller than bacteria. They are so small that 500 million rhinoviruses (which cause the common cold) could fit on the head of a pin. They can only be seen with a powerful electron microscope. Viruses are simple particles made of genetic material (called DNA or RNA) encased in a protective protein coat called a capsid. Some viruses have another coat or shell called an envelope. Take a peek at what's inside a virus.

Scientists differ about whether viruses are “alive” or not. Most say they are not really “alive” because they lack the characteristics of living things: they are not made of cells, they do not turn food into energy, and they cannot reproduce on their own. They can only multiply inside the cells of other living things such as animals, plants, bacteria, or fungi. Most cannot survive long unless they invade a living host. But once they are inside a host cell, they act very much alive.

How Does A Virus Infect You?

Using hand sanitizer

Viruses enter human bodies through the nose, mouth, and eyes, or through breaks in the skin. They can be passed by coughs, sneezes, insect bites, or even by food or water. Viruses attach to the host cells and get inside them. When a virus infects a cell, its genetic material sends that cell one simple message: Make more viruses! Eventually, the virus-filled host cell dies, sending out new viruses to invade more cells of the body.

Viruses follow some basic steps in order to force their host cells to make more viruses. These steps are called the Lytic Cycle.

  1. A virus attaches to a host cell. All viruses have some type of protein on their outside coats that "recognizes" the proper host cell for its type.
  2. The virus, or a virus particle, penetrates the host cell and releases its genetic instructions into the host cell.
  3. The injected genetic material gives instructions to the host cell's enzymes to make parts for more new virus particles.
  4. The new particles assemble the parts into new viruses.
  5. The new virus copies break out of the cell membrane and leave the host cell, ready to infect other cells.
Lytic cycle
Image courtesy of University of Barcelona

Viruses can reproduce quickly and make their host organism sick. You can think of them as microscopic hijackers or pirates: they invade your body, commandeer your cells, and force them to produce the virus. Viruses exist only to reproduce - to make copies of themselves. And they're very good at what they do!

You have probably heard of "computer viruses" and "viral videos." These terms do not refer to real viruses, but to things that spread rapidly like a virus. Computer viruses are a set of instructions that can "infect" its host computer. Viral videos spread quickly around the globe as people use technology to share them.

Immune System to the Rescue!

Sick Kid

Fortunately, your body has a defense against viruses. It is called the immune system. If a virus makes it past the barriers your body has - your tough skin, or the sticky mucus and little hairs called cilia lining your breathing tube - then this system takes over.

Here's how it works. The immune system is an organization of different types of cells, tissues, and enzymes working together to identify and eliminate all invading substances in your body. Each part of the immune system has its own specialized job. White blood cells are your main defense. They patrol your body. When they come across an antigen, a germ that doesn't belong there, they produce antibodies that work to fight against that particular antigen. Some antibodies destroy antigens while others make it easier for white blood cells to destroy the antigen.


Trillions of white blood cells are hard at work fighting enemy antigens. Sometimes, though, your body needs help from the medicines doctors give you. One of these medicines is called a vaccine. A vaccine is a tiny, weakened, or dead part of a germ that is injected into your body like a medicine. A vaccine is not trying to make you sick. Instead, it's just enough of a germ to get the body's immune system revved up and producing lots of antibodies. So if and when the real germ shows up, there will be lots of antibodies already in place to guard your cells from harm.

What if you do happen to get sick from a virus? Because viruses live inside cells, they are hard to treat with medicine. Antibiotics that treat bacterial infections, such as strep throat or ear infections, do not work on viruses. There are some antiviral medicines that block the entry of a virus into a host cell or interrupt the virus as it attempts to copy itself. But often, only the symptoms of a viral illness can be treated.

Girl using hand sanitizer

That's why it is so important to do everything you can to keep the virus from finding its way into your body. There are many steps you can take to prevent viral infections. Many viruses are passed from infected people to others through the air and through close personal contact. So stay away from people who are sick. Try not to touch your mouth, nose, or eyes, so that any germs that might be on your hands won't enter your body. Most important, wash your hands with soap and water for at least 20 seconds, several times throughout the day.


In 2020, a new coronavirus called SARS-CoV-2 spread across the globe. The disease known as COVID-19 infected many people in all areas of the world. In order to slow down the spread of the virus, people were asked to maintain distance from each other and to wear masks over their mouths and noses, and many schools, parks, restaurants, and sports activities were shut down. These actions made it harder for the virus to jump from one person to the next. If the virus can’t spread, fewer people get sick. Scientists worked to create coronavirus vaccines, and to develop medicines to help those who get sick. 

Carrona Virus

Kids usually don't get very sick from this virus, but you still have a role to play in protecting others. When there is an outbreak of the virus, you can help slow the spread and protect sicker people by washing your hands, wearing a face mask in public, and not gathering in crowds. Stay safe by following the guidelines of scientists and doctors.

Did You Know?

Agar Plate

There are hundreds of different kinds of viruses, and they're constantly changing. It's unlikely that you'll get sick from the same virus twice. That's because the immune system can remember its previous response to a virus attack, and if that foreign substance invades the body again, the immune system gets right to work. Here's a more complete answer to whether you can get sick from the same virus twice.

Plants get viruses, too! Here's an introduction to plant viruses along with some photos.

Virus Phage
Virus Phage

Is there anything good about viruses? Viruses invade host cells in living organisms, including bacteria. There are some viruses that only infect harmful bacteria cells. These viruses are called bacteriophages, which means "bacteria eaters." These virus phages can be helpful and important. Scientists are studying and finding ways to use phages to infect and destroy harmful types of bacteria that cause disease.

A virologist is a scientist or doctor who does research on viruses. An epidemiologist is a public health expert who studies diseases in populations of people. A microbiologist is a specialist in the world of tiny, microscopic creatures. Read more about these careers.

Learn more about microbes at Science Trek's Kingdoms of Life page and about your virus-fighting immune system at Science Trek's Blood page.

Top 10 Questions

November 2006

Thanks to Dr. Christine Hahn and Dr. Joseph Hornby for the answers.

  1. Why are scientists not sure whether or not a virus is a living thing?

    That's a very good question. It really boils down to the question, "What is life? What does it mean to be alive?" That's not really a purely scientific question. That's also a philosophical question. People have different opinions about what exactly is life. I think viruses don't meet what many scientists feel are the necessary minimum criteria for life. But there are other scientists that would say that because viruses can make copies of themselves, that means they are alive. That is one thing that's fun and exciting about science — it's not all cut and dried. There's room for argument. (From Rachel in Caldwell)

  2. What do viruses feed on?

    The major goal of a virus is to survive and to make copies of itself. Viruses really don't eat anything. They infect or enter into a cell called the host cell and then they basically use proteins and various parts of that host cell to make copies of themselves. Then they will leave that cell and go find more cells to invade and infect. So, they really don't eat anything. (From Ristan from Potlatch)

  3. How do you get rid of a virus?

    Often times your own immune system does the trick. Your own body will get rid of a virus that is trying to infect you. Some viruses can take a strong hold and you will need some medication for treatment. In some cases, we haven't found a way to get rid of certain viruses, so they may stay with you for the rest of your life, things like cold sores or canker sores. We have been able to eradicate one virus, smallpox. Compared to other viruses, smallpox was a little easier to get rid of. There's a vaccine that's very effective. Young kids probably never got this shot, but if they look at their parents' arms they can see a scar. It's a vaccine which causes a big scar. But a good vaccine was key to eradicating smallpox. Secondly, smallpox doesn't live outside a human so you don't have an animal host or place to get rid of too. Other viruses, like West Nile virus, one where birds can get it and mosquitoes can carry it, will be a bigger challenge to eradicate. (From Aubrey in Hailey)

  4. What's the difference between a virus and bacteria?

    Both viruses and bacteria are germs, of course, and they can both make you sick. Viruses though are much smaller and much simpler than bacteria. Bacteria are pretty complex little cells. They have a lot of intricate structures. Viruses are a very super simple little outfit, usually a shell with some genetic information inside. (From Mallory in Mrs. Wells' fourth grade class)

  5. How did we find the first vaccine for viruses?

    That one is a really interesting story. It was a very brave experiment on the part of a small boy. There is a similar disease to smallpox called cowpox. Cows carried this virus and it was noticed that milk maids, the young ladies who worked around cows a lot, didn't tend to get small pox as often as others. You always heard that term, "pretty as milk maid"? That phrase came into use because milk maids seemingly weren't scarred from small pox. So based on that theory, scientists took a little boy and purposely gave him cowpox and then, once he had recovered, exposed him to small pox. I can't imagine what mother agreed to that, but the little boy survived and did just fine. So, that was a huge step forward in proving that if you got exposed to one virus or something close to that virus, your body could build up an immunity and then you wouldn't get sick. The first actual use of what we call vaccinations, or vaccine was 60 or 70 years before that. In the early 1700s, there was a woman, Mary Montague, who was a writer from England and whose husband was an ambassador to what is now Turkey. She actually had small pox and was terribly scarred from it. She saw that the local women were doing something called engrafting, where they would take fluids from a person who had a certain type of small pox, one that isn't as deadly or damaging, and infected other individuals. She did that to her own son to protect him from what she had. (From Becky in Jerome)

  6. What is the most dangerous virus?

    Most likely, [something] like the Ebola virus, which tends to be very difficult to control once it infects an individual. It also has a fairly high mortality rate and can lead to quite a few deaths. I'm going to throw in what I think is a virus that's closer to home for most of us, in Idaho. The rabies virus, which exists in Idaho and is mostly in bats in Idaho, is very dangerous. It is a good reason to avoid bats and not touch them because if you're infected with that virus and you remain untreated, it almost always lead to death. Fortunately if you get bitten, you can get shots and prevent [yourself from] getting sick. But once people get sick with this virus, it's a really nasty one. (From Drew in Mrs. Kerr's class in Boise)

  7. Why isn't there a cure for the common cold?

    One of the major reasons why there isn't a real cure for the cold is that there are a large number of different types of viruses that actually cause what we call the common cold. The most common one goes by the name of rhinovirus, but there are many other types of viruses that do cause a cold. When rhinoviruses infect a cell, they cause errors when they copy themselves; and when they leave the cell they actually leave a little bit different than when they entered. That means your body has a difficult time remembering or recognizing that virus or the different virus. Viruses are constantly changing, so our body's immune system doesn't have a very good memory for each version of the rhinovirus. These viruses don't really cause an incredibly substantial infection, so we don't have a tendency to remember those as well so we can get sick all over again. (From Corey)

  8. How were viruses discovered?

    People knew of diseases that were caused by viruses for many hundreds of years, all the way back to when small pox was identified as a disease. Viruses were first identified in the late 1800s. Two people were looking at a virus that was infecting a tobacco plant. They found that if you tried to filter a solution that came from that plant, you could still infect another plant and cause it to get the same disease. We are able to show that, first of all, bacteria can be filtered and eliminated, but that viruses can actually get through that filter. That simple experiment allowed us to first know that there was something now called a virus, known as the tobacco mosaic virus, that infects a plant. The word "virus" is a Latin word for poison. (From Miranda in Pullman)

  9. How does a virus make you sick?

    Some viruses will actually cause damage to your own cells, which leads to illnesses like influenza, the flu virus. Other reasons why you end up feeling sick when you have a viral infection are because your body is trying to defend itself against the invading virus with symptoms like a fever, a runny nose, a cough and such. Those symptoms are actually a good thing because it's your body's way of trying to get rid of that virus and make you healthy again. (From Savannah and Brandon in Mrs. Hager's third grade class in Grangeville)

  10. How does a virus get into your body?

    They can come in through the nose or mouth, or sometimes through breaks in the skin, such as a mosquito bite. For most of us, it's through the mouth or even through rubbing your eyes. Also, viruses can get inside you when you breathe them in or by ingesting them when you eat food that's been improperly handled. Viruses like those soft surfaces and they can burrow in that way. (From Colton, Hugo, and Joe in Cathy Rankin's class)