Viruses are microscopic organisms that can infect hosts, like humans, plants or animals. They’re a small piece of genetic information (DNA or RNA) inside of a protective shell (capsid). Some viruses also have an envelope. Viruses can’t reproduce without a host. Some common diseases caused by viruses include the flu, the common cold and COVID-19.


What is a virus?

Viruses are small germs (pathogens) that can infect you and make you sick. They can infect humans, plants, animals, bacteria and fungi. Each one infects only specific types of hosts.

Viral infections in humans can cause no symptoms or make you extremely ill. Types of diseases they can cause include:

A virus is a small piece of genetic information in a “carrying case” — a protective coating called a capsid. Viruses aren’t made up of cells, so they don’t have all the equipment that cells do to make more copies of themselves. Instead, they carry instructions with them and use a host cell’s equipment to make more copies of the virus.

It’s like someone breaking into your house to use your kitchen. The virus brought its own recipe, but it needs to use your dishes, measuring cups, mixer and oven to make it. (Unfortunately, they usually leave a big mess when you finally kick them out.)

Viruses are also sometimes called “virions.”

Virus features

Viruses share some common features. Viruses:

  • Are made up of genetic material (RNA or DNA) and a protective protein coating (capsid).
  • Sometimes have another layer called an envelope around the capsid. Viruses without an envelope are called “naked viruses.”
  • Are similar to parasites — they need a host to reproduce. They’ll survive outside of a host until their capsid breaks down over time.
  • Are 100 to 1,000 times smaller than the cells in your body.


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What are the types of viruses?

Experts group viruses into categories — like family and genus — based on similar features, like size, shape and the type of genetic material they carry. Some common types of viruses that you might hear about include:

  • Influenza viruses.
  • Human herpesviruses.
  • Coronavirus.
  • Human papillomaviruses.
  • Enteroviruses.
  • Flaviviruses.
  • Orthopoxviruses.
  • Hepatitis viruses.

There are also some viruses that have unique qualities, like retroviruses and oncoviruses.

Influenza viruses (Orthomyxoviridae)

The Orthomyxoviridae family of viruses includes influenza A and B, which cause the flu. Strains of influenza A also cause avian flu (“bird flu”) and swine flu (H1N1).

Human herpesvirus (Herpesviridae)

Herpesviridae is a large family of viruses. They cause several types of illnesses, like oral and genital herpes, chickenpox, shingles, Epstein-Barr and cytomegalovirus (CMV).


Coronaviruses are a subfamily of viruses. SARS-CoV-2, the virus that causes COVID-19, is probably the most well-known coronavirus. But other types of coronaviruses cause mild illnesses, like a cold.

Human papillomavirus (HPV)

Human papillomaviruses are part of the Papillomaviridae family of viruses. They cause warts. Some types of HPV can lead to cancers.


Enterovirus is a genus (one level smaller than the group called a “family”) of viruses that infect your intestinal tract. Types of enteroviruses cause polio and hand, foot and mouth disease.


Viruses in this genus are often spread by mosquitoes. They cause illnesses like Zika, West Nile, dengue fever and yellow fever.


Viruses in the genus Orthopoxvirus cause blistering rashes. Mpox and smallpox are orthopoxviruses.

Hepatitis viruses

Though they don’t all belong to the same family or genus, hepatitis viruses all infect your liver. Hepatitis A, B and C are the most common.


Retroviruses are RNA viruses that use special proteins to make DNA. The virus then inserts its DNA into yours. Your cells read the viral DNA as if it were its own instructions. HIV and human T-lymphotropic virus 1 (HTLV-1) are retroviruses.


Oncoviruses are viruses that can cause cancer. Viruses that have been linked to specific cancers include:

  • HPV.
  • Epstein-Barr.
  • HIV.
  • Hepatitis B and C.
  • HTLV-1.
  • Human herpesvirus 8 (HHV-8).

Satellite viruses

Satellite viruses can’t reproduce without other, “helper” viruses. Most satellite viruses are found in plants.


Also just called “phages,” bacteriophages are viruses that specifically infect bacteria. Scientists are studying bacteriophage therapy as a potential way to treat bacterial infections that don’t respond to antibiotics.


How do viruses get into your body?

Viruses usually enter your body through your mucous membranes. These include your eyes, nose, mouth, penis, vagina and anus. Some viruses get in through a break in your skin or from a bite from a mosquito or tick.


How do viruses work?

Viruses have several steps to infecting cells and reproducing. They include:

  1. Attachment.
  2. Entry.
  3. Replication
  4. Assembly.
  5. Release.

Attachment and entry

Viruses can get inside of cells in three ways:

  • Receptor binding. Cells have receptors on the outside that can receive signals from proteins in your body. Think of them like doors. Some viruses trick cells into thinking they should be allowed inside, so the cells let them in the door.
  • Direct fusion. Some viruses attach directly to host cells to get inside.
  • Bacteriophages inject their genetic material into bacterial cells. The entire virus doesn’t need to get inside.

Replication, assembly and release

Once the virus or its genetic material is inside of a cell, it uses either a lytic cycle or lysogenic cycle to reproduce (some use both):

  • Lytic cycle. The virus uses the host cell’s machinery to make more copies of itself. Pieces of the virus assemble, wrapping up the genetic material in the capsid. Viruses make many copies of themselves this way. Eventually, there are so many copies of the virus inside the cell that it bursts. Those virions can now go and infect more cells.
  • Lysogenic cycle. Some viruses have a dormant, or silent phase. They get inside cells and then wait. Instead of setting up shop to cook in your kitchen right away, it’s as if they put their recipe into your body’s recipe book without you knowing it. The cells don’t realize the virus is there and continue to reproduce as they normally would. Each new copy of the cell also has a copy of the virus in it. Certain triggers can cause those cells to burst, spreading viral particles into your body that can infect other cells. Triggers could include stress, chemical signals or temperature changes.


What are the characteristics of viruses?

You can describe viruses based on a number of features, including:

  • What they look like (their shape and size).
  • Genome properties.
  • Structural proteins and whether or not it has an envelope.

Virus shapes

Viruses can look very different from each other. Scientists often described them by shape. Types of virus shapes include:

  • Icosahedral or polyhedral. This is a geometric shape with many sides, similar to a soccer ball. Most viruses that infect people are icosahedral.
  • Helical. This virus shape looks like a cylinder. Its genetic information is coiled up like a spring inside.
  • Spherical. Spherical viruses are helical or polyhedral viruses that have an envelope around them. They’re shaped mostly like a ball.
  • Complex. Complex viruses combine more than one shape. Viruses that infect bacteria have a polyhedral “head” connected to a helix “body.”

Virus size

All viruses are very small — too small to see without a strong microscope. If you measure them under a microscope, most are between 20nm (nanometers) to 400nm. For comparison, the smallest viruses are about 2,000 times smaller than a grain of sand. They’re about 100 to 1,000 times smaller than the cells in your body.

But their sizes can vary a lot. For instance, the measles virus is about five times larger than Zika virus. Viruses also have varying weights (molecular weight).

Genomic properties of viruses

The information stored in the virus — its genetic material — is either DNA or RNA. DNA is like the instruction manual for how to build the virus. RNA is like the translation of the instructions in a language that the cell machinery can read and make into proteins. Viral DNA or RNA can be:

  • Linear or circular.
  • Positive-sense or negative-sense. RNA that’s positive-sense can be used as instructions to make more virus parts without any additional steps. Negative-sense RNA viruses need special proteins (enzymes) to create positive-sense RNA before viruses can make more copies of themselves. This is also called “plus-strand” (or “positive-strand”) or “minus-sense” (or “negative-strand”). Most DNA viruses are positive-sense.
  • Single-stranded or double-stranded. DNA viruses can have their genetic material in a single string of instructions (ssDNA), or two sets that are paired together (dsDNA). (Human DNA is double-stranded.) RNA viruses are usually single-stranded, though there are some double-stranded RNA viruses.

Structural proteins

The structural proteins of a virus make up the capsid, or protective coating. They can also make up the envelope, if there is one, and any structures that stick out from it that help it enter cells (like the spike proteins of coronaviruses).


Are viruses living or nonliving?

Viruses aren’t living organisms. But there’s some debate over this. Generally, biologists don’t consider viruses to be alive because they can’t perform the functions that living organisms do. For instance, they can’t convert food into energy (metabolism) and they can’t live or reproduce without a host cell.

On the other hand, they can reproduce in the right host cell and they evolve over time to survive. Plus, they can damage and destroy host cells to do so. Because of this, many consider them a “gray area” between living and nonliving things.

Conditions and Disorders

What are some diseases caused by viruses?

Viruses can cause respiratory illnesses, “stomach flus,” sexually transmitted infections (STIs), skin conditions and many other kinds of illnesses. Some commonly known viral infections include:

A note from Cleveland Clinic

Viruses come in many shapes and sizes. Some look like spiky balls and others look like spiders with a space helmet on. All of them need a host to reproduce, but not all of them make you sick. In fact, biologists estimate that there could be trillions of harmless viruses living inside you right now. And while only some viruses cause infections in people, the ones that do make pretty terrible houseguests.

Medically Reviewed

Last reviewed on 03/29/2023.

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