Virus

Viruses are small germs that can infect you and make you sick. They’re similar to parasites in that they need a host to reproduce. Each one infects only specific types of hosts. They can infect humans, plants, animals, bacteria and fungi.

Unlike humans, viruses aren’t complete organisms with cells, tissues and organs to perform the functions they need to reproduce. Instead, a virus is a small piece of genetic information (DNA or RNA) in a “carrying case” — a protective coating called a capsid.

Since they aren’t made up of cells, viruses don’t have all the equipment that cells do to make more copies of themselves. They can live outside of a host, but not reproduce, for a short period of time until they break down. So, they carry instructions with them and use a host cell’s equipment to reproduce.

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, viruses usually leave a big mess while your body tries to kick them out. And instead of a sink full of dishes, they make you sick.

Viruses are also sometimes called “virions.”

How do viruses infect you?

Viruses infect your cells and reproduce through several steps:

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

Viruses usually enter your body through your mucous membranes — like your eyes, nose, mouth and genitals. Some viruses get in through a break in your skin or a bite from an insect or animal.

Once there, the virus can enter your cells in two ways:

• Receptor binding: Cells have receptors on the outside that can get signals from proteins in your body. Think of a receptor like a lock on a door. Some viruses trick receptors into thinking they have the right key, so the cells let them in the door.
• Direct fusion: Some viruses attach directly to host cells to get inside.

To infect bacteria, phages inject their genetic material into bacterial cells. The entire virus doesn’t need to get inside.

How do viruses reproduce?

A virus uses either a lytic cycle or a lysogenic cycle, or both, to reproduce once it’s inside of a cell.

Lytic cycle

The virus uses the host cell’s machinery to make more copies of itself. Pieces of the virus assemble, wrapping up their RNA or DNA into a 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 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 — like stress, chemical signals or temperature changes — make those cells burst. They spread viral particles into your body that can infect other cells.

Virus parts

Viruses are made up of:

• Genetic material: This is either DNA or RNA that tells cells how to make more viruses. 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.
• Capsid: This is a protective layer around the genetic material. The capsid or the envelope gives a virus its shape.
• Envelope: Some viruses have an extra layer around the capsid, called an envelope. Viruses without an envelope are called “naked viruses.”
• Structural proteins: These are unique proteins that make up the capsid and, if there is one, the envelope. They also make up any structures that stick out from the virus that help it enter cells (like the spike proteins of coronaviruses).

Virus shapes

Scientists often describe viruses by their shape, which can 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. For example, viruses that infect bacteria have a polyhedral “head” connected to a helix “body.”

Genomic properties of viruses

You might hear a description of a virus that’s based on features of its DNA and RNA. Some descriptions could include linear or circular, positive-sense or negative-sense, and single-stranded or double-stranded.

Linear or circular

Linear DNA or RNA has two free ends, like a piece of thread. Circular DNA or RNA makes a ring without free ends.

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 they can make more copies of themselves. This is also called “plus-strand” (or “positive-strand”) or “minus-strand” (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 in two sets that are paired together (dsDNA). Human DNA is double-stranded. RNA viruses are usually single-stranded, but some are double-stranded.

How big are viruses?

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

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).

Types of viruses

Viruses can cause respiratory illnesses, the “stomach flu,” sexually transmitted infections (STIs), skin conditions and many other kinds of illnesses. 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:

• Coronaviruses: Coronaviruses are a subfamily of viruses. SARS-CoV-2, the virus that causes COVID-19, is a coronavirus. Other coronaviruses cause common colds.
• Enteroviruses: Enterovirus is a genus of viruses that infect your intestinal tract. One type of enterovirus causes polio, while another type causes hand, foot and mouth disease.
• Flaviviruses: Viruses in this genus are often spread by mosquitoes. They cause illnesses like Zika, West Nile, dengue fever and yellow fever.
• 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.
• 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.
• Human papillomaviruses (HPV): Human papillomaviruses are part of the Papillomaviridae family of viruses. They cause warts.
• Influenza viruses (Orthomyxoviridae): The Orthomyxoviridae family of viruses includes influenza A and B, which cause the flu and avian flu (“bird flu”).
• Orthopoxviruses: Viruses in the genus Orthopoxvirus cause blistering rashes. Mpox and smallpox are examples.
• Retroviruses: Retroviruses (in the family Retroviridae) 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.

Other special types of viruses include:

• Oncoviruses: These are viruses that can cause cancer. Viruses that have been linked to specific cancers include HPV, Epstein-Barr, HIV, and hepatitis B and C.
• Satellite viruses: Satellite viruses can’t reproduce without other, “helper” viruses. Most satellite viruses are found in plants.
• Bacteriophages: 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.

Are viruses alive?

There’s some debate over whether viruses are alive or not. 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 experts consider viruses a “gray area” between living and nonliving things.