What is a virus?
Viruses are tiny, but they pack a big punch. Viruses spread and multiply fast, causing some of the most common and contagious diseases in the world. If you've ever had a rash, flu, or warts, you've probably hosted a few viruses. Actually, you've hosted billions of viral particles.
A virus consists of a small amount of genetic material blueprint (DNA or RNA) inside a fatty-lipid-protein case. The genetic blueprint of some viruses, such as herpes viruses, is DNA, while the genetic material of other viruses, like HIV or the flu virus is RNA. As you know, if you've completed the Zero to Genetic Engineering Hero journey, or gone through the What is DNA? Simulator, DNA, and RNA store information for making proteins, which in turn build a complete organism.
Coronaviruses like the nCov-19, more recently called SARS-CoV-2, which leads to the COVID-19 disease, are single-stranded RNA viruses, which means that their genome consists of a strand of RNA (rather than DNA).
It's important to remember that a virus is not a cell. Viruses are a thousand times smaller than even a bacterium and can exist in a wide variety of shapes and forms. They are often described as parasites because they must be inside a living cell to reproduce, and the relationship with its host is parasitic (the guest benefits at the expense of the host). Just like a computer virus is only able to be "alive" when it is inside a computer's operating system, a biological virus is inanimate until it gets into a living cell and has access to the cell's proteins machinery and DNA. With the cell's machinery, the virus can "unpack" and begin copying its nucleic acid, and transcribing and/or translating proteins.
What viruses don't do:
Viruses are not made of cells; they do not obtain or use energy to run metabolic activities (they do not have a metabolism because they are just particles and not cells). They do not grow in size or develop over a lifetime from a young virus to a mature virus. They are not able to respond to a stimulus in their environment because they are not alive, and they do not maintain homeostasis as living cells do when they expel waste or take in food and water. This is mainly because viruses themselves lack the "cellular machines" to copy their nucleic acid blueprint as well as to read that blueprint to make proteins and other important molecules. They rely completely on the machinery of the cells they are infecting.
What viruses do:
Viruses do have genetic blueprints that change in time (e.g., mutate) and give the virus a new characteristic that might allow it to have an advantage in their environment. This natural selection happens when a slightly mutated virus is better able to infect a target organisms' cells and replicate to produce more of itself, which can further infect other cells. In other words, a slight change in the virus DNA or RNA blueprint can make it, so the virus is better able to infect new cells and thus "survive," which results in the evolution of the virus population over time. Also, viruses "replicate," but they are not capable of doing this independently, and they do not divide as cells do using mitosis or binary fission. Making more viruses is called replication rather than reproduction because they take over a living cell and use the cell's existing machinery to make copies of themselves by assembling more pieces of nucleic acids and fatty-lipid-protein coats. These new virus particles leave the host cell either one at a time or all at once to go infect a new host cell and make more copies.
Bacteria can catch viruses too!
Did you know that sometimes even bacteria can be infected by viruses? For example, when you do the genetic engineering experiment from the Engineer-it Kit or paint bioart with the Canvas Kit, you are using a lab-strain of E. coli that is safe for you and the environment. One of the differences that this lab strain of E. coli has is that it was changed by scientists so that they no longer have a "lambda phage" infection. Lambda phage is a virus that infects E. coli and becomes part of its genome. Yes, even bacteria can get viruses and catch a cold! The original version of the E.coli strain called K12 has the lambda phage in its genome. Under the right conditions, the virus can become active, kill the cells, and spread to others. Lab strains of K12 E. coli no longer have this infection, and you do not have to worry about your bacteria catching a cold during your experiments. Oof!
How do viruses spread?
Viruses spread in many ways and different types of viruses will spread in different ways. Each species of virus will rely on a particular method to spread. For example, plant viruses will often be spread by insects and microorganisms from plant to plant. Some of the vaccines that affect humans and animals will spread through infected bodily fluids while others will spread through the air in droplets of moisture released when infected people sneeze or cough. This is the case with the flu and the coronavirus.
See how viruses spread in the human population by using this simulation of the Coronavirus spread at the Washington Post.