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:
Respiratory illnesses.
Diarrhea and vomiting.
Sexually transmitted infections (STIs).
Skin conditions.
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.
Function
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:
Attachment.
Entry.
Replication
Assembly.
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.
Anatomy
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.