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Learning goals

What is a virus made of?

How do they reproduce?

Classifying viruses by how they affect us.

What is a virus made of?

A virus is a small particle that only replicates inside cells of other organisms.

Virus structure

Viruses contain genetic material – DNA or RNA.

They are surrounded by a capsid (protein capsule)

Important facts

A virus, by itself, shows no signs of life.

Viruses can infect all types of life – animals, plants, even bacteria.

They are found in every ecosystem on Earth

They are the most abundant type of biological entity.

How do they reproduce? 

A living cell is like an office of blind workers that make photocopies.

Look at the picture below: What would happen if someone slipped in an incorrect page into the photocopy file?

Since these workers can’t see that this page is incorrect then they’d just photocopy it along with everything else.

Now – what if those copies were directions on how to run the office?

The people running the office would following the normal directions, but also follow the off-topic directions.

This could harm how the office works.

DNA RNA blueprint analogy copier

Viruses spread exponentially

How does the likelihood of death from any common cause compare to the likelihood of death from something that spreads exponentially? The important difference is that for any other cause of death, that cause is (a) usually not transmissible, and (b) the rate of death stays (more or less) the same over time.

But for deaths caused by a virus the situation is different – (c) it is transmissible from one person to another, and (d) the number of people infected grows exponentially over time.

Animation: Global Deaths Due to Various Causes and COVID-19

Global Deaths Due to Various Causes & COVID-19 (YouTube video)

Methodology and sources for the animation


Classifying viruses by how they affect us

Viruses harmful to people

Ebola, Hepatitis, rabies, mumps, measles, rubella, smallpox, HIV, polio, chicken pox, influenza, types that cause pneumonia, etc.

Also, some viruses are slightly harmful to people, but often just an annoyance. Herpes.

See Vaccines and herd immunity.

Viruses helpful to people

Some bacteriophages can kill harmful bacteria

Cancer-fighting viruses win approval US regulators clear a viral melanoma therapy, paving the way for a promising field with a chequered past.

Friendly Viruses Protect Us Against Bacteria By Beth Skwarecki

Viruses: You’ve Heard the Bad. Here’s the Good

Viruses: They’re Not All Bad, Says Expert. By Brian Stallard

Move over bacteria! Viruses make their mark as mutualistic microbial symbionts. By Marilyn J. Roossinck

Viruses that don’t affect human health

“Most viruses do not infect and cause disease—they are not pathogens…. [H]umans are always infected with viruses, most of which rarely, if ever, become symptomatic.

– from Viruses May Be Harmful, Helpful, or Passengers by Marco Cáceres

Structure of viruses

Viruses are not cells.

They have a completely different structure.

Most viruses are very tiny – hundreds of them could fit inside a single cell.

HIV Virus infecting a cell

HIV infecting a human cell. Credit: NIH Image Gallery

Giant viruses

We recently discovered a few giant viruses – almost as large as a cell! They are so different from other viruses that scientists hypothesize that they are not related to regular viruses.

They may have evolved from an earlier form of life that no longer exists.  See pandoraviruses.

Study adds to evidence that viruses may be considered alive

Size mimivirus mycoplasma bacterium HIV archaean

Image from New Scientist magazine

Are viruses alive?

Resources for discussion:

Ernst Schrodinger gave a series of lectures in Dublin in 1943 entitled ”What is Life?” He asked, “How can the events in space and time which take place within the spatial boundary of a living organism be accounted for by physics and chemistry? His conclusion (which seems a pretty good general definition) is that Life involves creating order from disorder: ie manipulating energy to reverse entropy within a boundary. – Quora: Are-viruses-living-or-non-living?

Ask a biologist: Are-viruses-alive?

Sciencealert.com: Are-viruses-alive?

Are Viruses Alive? Although viruses challenge our concept of what “living” means, they are vital members of the web of life.
By Luis P. Villarreal on August 8, 2008. Scientific American

E. P. Rybicki. 1990. The classification of organisms at the edge of life, or problems with virus systematics. S Aft J Sci 86:182–186 Advances in Virus Research. 394 pages

Defining Life: The Virus Viewpoint. Patrick Forterre

Virus lifecycle

Here is an example of a virus lifecycle.

Virus Lifecycle

Advanced topics

Could plant viruses infect humans? Not as far as we know, but the possibility exists.

Can Plant Virus Infect Human Being? Bikash Mandal and R. K. Jain

Evolution of viruses

How viruses originated – sources to be added.

Over time, many viruses evolve to become less immediately dangerous to humans. Why In “Evolution from a virus’s view” (*) we read:

There may be an evolutionary trade-off between virulence and transmission. Consider a virus that exploits its human host more than most – and so produces more offspring than most. This virus does a lot of damage to the host ( highly virulent.) From the virus’s perspective, this would, at first, seem like a good thing: Extra resources mean extra offspring, which generally means high evolutionary fitness.

However, if the viral reproduction completely incapacitates the host, then the whole strategy could backfire: the illness might prevent the host from going out and coming into contact with new hosts that the virus could jump to. A victim of its own success, the viral lineage could go extinct and become an evolutionary dead end. This level of virulence is clearly not a good thing from the virus’s perspective.

(*) Understanding Evolution. 2018. University of California Museum of Paleontology, Jan ’18.


However, this not mean that evolution makes all viruses safer over time. This only means that it makes them less immediately deadly.  These viruses could still hurt people slowly, over many decades.

Book: Plague Time: The New Germ Theory of Disease, by Paul Ewald

Paul Ewald: Infectious Disease and the Evolution of Virulence. PBS WGBH

The Big Idea That Might Beat Cancer and Cut Health-Care Costs by 80 Percent. Discover Magazine.

bacteriality.com Interview with evolutionary biologist Paul Ewald

theatlantic.com: A New Germ Theory. The dictates of evolution virtually demand that the causes of some of humanity’s chronic and most baffling “noninfectious” illnesses will turn out to be pathogens — that is the radical view of a prominent evolutionary biologist.


The COVID-19 Pandemic

Coronavirus disease 2019 (COVID-19) is a contagious respiratory and vascular disease.

It is caused by a virus called severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2).

Covid-19 spread in Italy as early as September 2019, while the first case was identified in Wuhan, China in December 2019.

While most people have mild symptoms, some people develop acute respiratory distress syndrome (ARDS). ARDS can be precipitated by cytokine storms, multi-organ failure, septic shock, and blood clots.

Longer-term damage to organs (in particular, the lungs and heart) has been observed. There is concern about a significant number of patients who have recovered from the acute phase of the disease but continue to experience a range of effects—known as long COVID—for months afterwards.

COVID-19 may spread via small droplets/aerosols, as an infected person breathes, coughs, sneezes, sings, or speaks. It could be spread through contaminated surfaces.

It can spread as early as two days before infected persons show symptoms (presymptomatic), and even spread from asymptomatic (no symptoms) individuals. People remain infectious for up to ten days in moderate cases, and two weeks in severe cases.

Preventive measures include social distancing, quarantining, ventilation of indoor spaces, covering coughs and sneezes, hand washing, and keeping unwashed hands away from the face. The use of face masks minimizes the risk of transmissions.

(intro adapted from Wikipedia, Coronavirus disease 2019)

How to deal with a viral pandemic


How do viruses spread? Airborne vs non-airborne

Unmasking mask myths

Face masks – which ones really work?

Further resources to explore

Viral Attack (comic book)

Story Behind the Scenes    Click here to read the comic: “Viral attack!”

For Teachers

Learning Standards

2016 Massachusetts Science and Technology/Engineering Curriculum Framework

HS-LS4-4. Research and communicate information about key features of viruses and bacteria to explain their ability to adapt and reproduce in a wide variety of environments.
College Board Standards for College Success: Science

LSH-PE.5.4.4 Give examples, using evidence gathered from print and electronic resources, of instances when viruses are linked to cancer. Explain, based on knowledge of viral gene insertions and of the relationship among DNA, proteins and traits, how a viral insertion into DNA can cause cancer.

LSH-PE.5.4.5 Give examples, using evidence gathered from print and electronic resources, of the potential of using viruses for curing genetic diseases via gene therapy. Make a claim about, and justify, based on knowledge of viral DNA and viral insertions, why some viruses are appropriate for this application.

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