What Are Viruses?

Common tiny microbes can cause infections and illness

Viruses are tiny microbes made of a protein shell with genetic material inside. They must rely on infecting other cells to produce more viruses. A virus is too small to see with the naked eye. They’re found everywhere.

A virus tailored to humans can invade your body, take over your cells, and make you sick. Viruses aren’t considered living organisms themselves because they must use their hosts’ cells to copy their genes and create new viruses.

Viruses come in many different types. Not all viruses make people sick. Sometimes, your immune system can fight them off without you even knowing.

This article will cover what viruses are, the different types of viruses, how viruses work, some common virus examples, virus symptoms, treatment, and prevention.

Computer generated image of coronavirus

Yuichiro Chino / Getty Images

What Are Viruses?

A virus is a tiny non-living microbe. It must infect a cell to make copies of itself inside. They have a genome of DNA or RNA inside a shell of proteins. Some of them also have a layer of fats around the protein shell.

Viruses infect humans, animals, plants, bacteria, amoeba, and fungi. If there’s a living thing, there’s probably a virus that infects it. The living thing infected by the virus is called its host.

Because viruses rely on other living things to make new viruses, don’t make their energy, and don’t grow, they are not typically thought to be alive. There is still some controversy over this because there are other living things that we would traditionally think of as “alive” that rely on a host for energy, replication, or some other factor of their being. These are called obligate intracellular parasites. 

Types of Virus

More than 200 viruses are known to infect humans from 26 virus families. More are discovered every year.

There are many ways to classify viruses.

  • The type of genetic material (RNA or DNA) they have
  • If there are one or two strands of genetic material (double or single-stranded)
  • Other structural features of the genome, including which direction the strands are read, if they have multiple pieces, and other structural differences
  • Genome size, which can vary significantly from virus to virus (typically, the viral genome is 7,000–20,000 base pairs long)
  • The shape of the outer protein shell or how it looks under the microscope (icosahedral or helical)
  • If there’s an outer layer of fat around the shell
  • On average, size ranges from around 20–100 nanometers (nm) in diameter, generally 100 to 1,000 times smaller than the cells they infect (though some viruses are nearly 1000 nm long)
  • How the virus copies its genes
  • The host the virus infects (humans, mammals, insects, bacteria)
  • The parts of the body the virus infects (for instance, respiratory viruses infect the lungs, throat, and sinuses)

What Are 5 Common Viruses?

Some common viruses are those that cause:

How Do Viruses Work?

Viruses use proteins on their outer shell or fatty layer to find and stick to their host cells. They use the proteins to put their genes inside the cell. When there, the genes use the cell’s machinery, make copies of the genes, and build more virus protein shells.

The cells put together new viruses and release them. They go on to infect many more cells, spreading the infection.

How Are Viruses Transmitted?

Viruses can’t move on their own. They depend on host actions to transmit their infections. 

Infections are transmitted in many ways, including through:

  • Droplet transmission: Viruses can spread from up to 6 feet away from a person with a respiratory viral infection in the air in tiny droplets of spit or snot, and these droplets can also settle on surfaces.
  • Airborne transmission: Some viruses can be airborne, with coughs or sneezes producing particles smaller than droplets that can remain suspended in the air for a longer time and travel farther.
  • Indirect contact transmission: You may touch a surface that has a virus present (the source may be saliva, mucus, feces, or other bodily fluids) and then touch your eyes, nose, or mouth. Surfaces can include tabletops, doorknobs, or casual physical contact such as a handshake.
  • Direct contact transmission: Close physical contact (including sexual contact) with bodily fluids or open sores may transmit a virus. Bloodborne viruses may also spread through shared needles.
  • Vector-borne transmission: Some viruses are transmitted from animals or insects to humans. This may be through bites.
  • Fecal-oral transmission: Food or water can be contaminated by viruses present in feces, such as from sewage or a food handler not washing their hands after using the restroom or changing a diaper. Eating food or drinking water with a virus may transmit it.

What Are the Symptoms of a Virus?

When a virus infects you, it may cause symptoms. Some of these symptoms are from the virus, and some are from the immune system fighting back. Depending on what cells of the body are infected, the virus may cause different symptoms. 

Some general symptoms of a viral infection include:

  • Fatigue, fever, chills, sweats, body aches, etc. 
  • Gastrointestinal distress, including nausea, vomiting, stomach cramps, and diarrhea from norovirus.
  • Respiratory symptoms include congestion, sneezing, coughing, and shortness of breath from COVID-19.
  • Rashes or lesions on the skin, for example, chickenpox.
  • Organ-specific symptoms, including jaundice for liver failure from hepatitis C.

How Are Viruses Treated?

Time is the best treatment for most viral infections, including day-to-day colds and stomach flu. Time, rest, and hydration will let your body take care of most viral invaders. Additionally, most people can take fever-reducing medicines like Tylenol (acetaminophen) or Advil (ibuprofen) to help with fevers or pain.

In some cases, virtual infections are treated with drugs called antivirals. For instance, if you have the flu, your healthcare provider may prescribe the antiviral Tamiflu (oseltamivir phosphate)  to help you get better quicker. 

Antibacterial medicines won’t help you get better from a virus. Taking antibiotics when you don't need to also increases the likelihood of creating superbugs that these drugs can no longer affect. So unless your healthcare provider has confirmed that a bacterial infection is causing your symptoms, avoid taking antibiotics.

Can Viruses Be Prevented?

Vaccines can prevent some viral infections. Vaccines are drugs that prime your body to fight back when it comes into contact with an invading germ. Some vaccines you need to get only once, while others you may need to get every year, like the flu vaccine.

Vaccines typically make the viral infection milder, less dangerous, and make you feel less sick if you do get an infection. You may not even notice you’ve been exposed to a virus if you’ve been vaccinated against it.

Over a dozen vaccines can help you fight off viral infections. People typically get many vaccines during infancy and childhood. Others may be useful during pregnancy or while traveling internationally. 

The other significant way to protect yourself against viruses and other germs is to practice good hygiene.

The first step to good hygiene is to wash your hands often or use a hand sanitizer with at least 60% alcohol. When washing your hands, use warm water and soap for at least 20 seconds. 

This is especially true when you’re: 

  • Eating or preparing food
  • Around someone sick
  • Using the toilet
  • Sick yourself and coughing or sneezing
  • Around animals

Otherwise, you can avoid catching a virus by avoiding sick people, keeping safe around food handling, cooking, and storing, and regularly cleaning and disinfecting surfaces and objects.

You can slow airborne viruses that spread through droplets by wearing a respiratory face mask, especially in a crowded indoor space.

Generally, staying home if you are sick is a good idea so you don’t risk transmitting your germs to someone else. 


A virus is a tiny non-living microbe. It can break into a cell and take over. It makes copies of itself when inside. Its genome is either DNA or RNA and is inside a protein shell. Some have a layer of fat around the protein shell.

Viruses infect humans, animals, plants, bacteria, amoebas, fungi, and any other living organism they can find. They can cause disease. Some viruses cause sickness, like common colds, the flu, and food poisoning.

Symptoms of a viral infection include fever, pain, nausea, diarrhea, cough, congestion, shortness of breath, rashes, warts, and many others. 

The best treatment for most everyday viruses is rest, hydration, and time. In some cases, antivirals may help speed healing. Vaccines can prevent some viral infections.

Otherwise, general hygiene help reduces the number of viruses you catch. Hygiene includes washing hands, avoiding sick people, and wearing a mask.

14 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Jennifer Welsh
Jennifer Welsh is a Connecticut-based science writer and editor with over ten years of experience under her belt. She’s previously worked and written for WIRED Science, The Scientist, Discover Magazine, LiveScience, and Business Insider.