This illustration shows a 3D graphic representation of a spherical-shaped mumps virus studded with different proteins.
What are Viruses?
The word "virus" comes from the Latin word for poison. Initially the term virus was used to describe all diseases that were transmitted between humans, but that was before we knew that bacteria could also cause disease. Now we know that viruses...
...are obligate intracellular parasites, meaning that they need to infect a host cell to make copies of themselves, and in doing so, harm the host.
...have genomes made of either DNA or RNA (not both) that is protected by a protein coat. If they have an extra lipid bilayer membrane around the protein coat, we call them "enveloped" viruses; if not, they are called "naked" viruses.
...are assembled using the host cell's own protein synthesis "machinery" rather than growing and dividing like cells do.
...are smaller than bacteria, but still vary in size. Use this "Scale of the Universe" link to see just how small viruses are!
...are great models for studying genetics and molecular biology. In fact, DNA replication was first worked out in viruses, and the first cancer causing genes were discovered in viruses and then used to find similar genes in humans.
We have also learned that there are viruses called "bacteriophages" that only infect bacteria (prokaryotes), but we are going to focus on eukaryotic viruses, since those are the viruses that infect us! Look at the diagram below to see the different types (shapes) of viruses. If you want to read more about what viruses are, use the "Explainer: What is a Virus?" link below the diagram.
How do Viruses Spread?
Once a virus successfully infects one of your cells, it "reprograms" it, turning your cell into a viral factory. The diagram below shows the general steps for how a single virus can create thousands of viruses in a single cell before spreading to other cells within your body (and eventually other people)!
1. The virus attaches to a receptor on your cell.
2. The virus enters your cell.
3. The protein coat of the virus breaks apart, releasing the viral genome into your cell.
4. The viral genome is replicated using your cell's own proteins.
5. The virus uses your cell's ribosome to make new viral proteins. (If the viral genome is DNA, then it needs to be translated into RNA before it can be "read" by your ribosomes.)
6. New viruses are assembled.
7. The new viruses break out of the cell and begin infecting other cells in your body.
If you are interested in learning more about the protein structure of viruses, or specifically the coronavirus causing the COVID-19 pandemic, use the "Fold it" game linked below. The concept of the game is to design a protein that could be used against Coronavirus. The most successful designs can be made into a drug to help fight the pandemic.
Once a person has become infectious, they can spread the virus to other people, either directly, or through fomites (inanimate objects like doorknobs that the virus can "survive" on). Humans can also get viral infections from animals. This is known as vector transmission. In many cases, these viruses cannot spread from human to human; they must be carried by an animal (e.g., Yellow fever and the West Nile Virus). However, there are also viruses that have "jumped" from animals to humans, and then spread from human to human. These are called zoonotic viruses. Despite what it seems like, these viruses aren't actually new. As humans delve further into untouched habitats and interact more with animals, we encounter more and more viruses. Occasionally, the viruses we encounter have mutations that allow them to infect humans or other animals.
Watch the videos below to learn more about how viruses spread.
How do we fight off viruses?
There are a few ways we fight off viruses, but let's start with the most basic: washing your hands! I know, I know...this is redundant right? Well, there is a reason why this always gets brought up. Watch the video below to learn why soap works, and the best way to wash your hands.
Note: At the end of the video there is a demonstration of how to extract your own DNA using household materials. You can find instructions for how to do that here.
If we do come into contact with pathogens such as viruses our body's immune system steps in. In the most basic sense, our immune system works in four steps:
1. Prevent infection - Your skin and mucus are great at this, but sweat and tears are also good barriers.
2. Recognize the pathogen - Specialized cells recognize the unfamiliar (foreign) pathogens in your body.
3. Eliminate the pathogen - More cells step in to "kill" the invader.
4. Remember the pathogen - Once your body has recognized and targeted the pathogen, long-lasting memory B cells and T cells are made. Their job is to respond immediately if you are re-infected by that pathogen.
Although these steps are presented as simple, each of them is actually quite complicated, and involves a variety of specialized cells. If you want to learn more about the immune response in detail, watch one or both of the videos below.
Finally, we have figured out a way to take advantage of our immune response and prevent infections before they ever happen. This is achieved with vaccines. Watch one or both of the videos below to learn more about vaccines and how they work.
Challenge/Extension:
Below is a short list of viruses/diseases. Choose one or more of them to research. What specific virus, or type of virus, is involved? What are the symptoms of the virus? How do we treat people with the virus? How does the virus spread? What else can you learn about the virus or disease?
Respiratory Viruses: Rhinovirus, Human Coronavirus, Adenovirus, Orthomyxoviruses, Paramyxoviruses
Enteric Viruses: Rotavirus, Norwalk virus, Poliovirus, Coxsacklevirus, Echovirus
Arboviruses: Yellow fever, Dengue, West Nile, St. Louis Encephalitis, Western Equine Encephalitis, Chikungunya
Zoonotic Viruses: Hantavirus, Rabies, Ebola, Lassa Fever, Sin Nombre, Monkey pox
Bonus
Below are a variety of additional videos with helpful information about viruses in general, and the corona virus specifically.