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.
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.”
Viruses share some common features. Viruses:
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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:
There are also some viruses that have unique qualities, like retroviruses and oncoviruses.
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 papillomaviruses are part of the Papillomaviridae family of viruses. They cause warts. Some types of HPV can lead to cancers.
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:
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.
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.
Viruses have several steps to infecting cells and reproducing. They include:
Viruses can get inside of cells in three ways:
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):
You can describe viruses based on a number of features, including:
Viruses can look very different from each other. Scientists often described them by shape. Types of virus shapes include:
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).
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:
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).
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.
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.
Last reviewed by a Cleveland Clinic medical professional on 03/29/2023.
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