Close your eyes and picture something alive. Perhaps you’re envisioning a person or a pet dog? Now, picture something that is not alive. Maybe, this time you’re thinking of a rock or a lightbulb.
Seems obvious which of your conjured images are alive and which are not alive, right?
Can you describe what makes something alive?
Will those characteristics of being alive still apply to a barnacle, a flower, or the E.coli that gives you food poisoning?
It turns out that describing the characteristics of living things requires us to dig a bit deeper than behaviors and features of an organism. As scientists we need to really think about the processes that all living things can do. However, even once we’ve done that, we can still run into trouble. This is because even scientists don’t all agree on what behaviors making something alive.
Are viruses alive?
Review some fundamental characteristics of living things. Challenge yourself to apply them to viruses (or not!). Then, think about why it even matters to classify a virus as a living thing or an inanimate object.
What makes something alive? What must it be able to do? Write down a list of the most essential elements. Check your list by seeing if it works to describe a wide variety of living things and excludes a wide variety of non-living things.
Describe viruses based on your set of criteria. Does this suggest they are alive or not?
What about a parasite? Or an obligate symbiote (an animal that MUST live in connection with another)?
Now let’s learn a bit more about viruses…
Relatively recently, scientists were surprised to discover viruses so large they are the same size as some bacteria. Might this large size give them the features of other living things? Read this article about giant viruses and see what you think!
Scientists are also puzzled by viruses that infect other viruses. Can a nonliving thing invade another nonliving thing or should we reframe what we think of as being alive? Read more about virophage and make up your own mind.
Did anything in the articles change your mind? Do you want to modify your list of what makes something alive or adjust your understanding of viruses in some way?
Here are a few things to consider
Characteristics of living things
There are generally several factors considered in whether something is alive, including:
- Can it obtain nutrients?
- Can it extract energy from these nutrients (respiration)?
- Can it excrete waste?
- Can it maintain a consistent internal environment (homeostasis)?
- Can it reproduce and make offspring?
- Can it sense and respond to the environment?
Which of these apply for viruses and which don’t (or don’t always)? Do you agree that all of these should be required to define something as alive?
Does it matter what the virus is “doing?”
Viruses are sometimes thought of differently when on their own versus in a host.
A virus on its own is a neatly packaged particle, believed to be static in nature. It doesn’t take in nutrients or excrete wastes.
Think of it like a package sent from Amazon on its way to your house. But, when that package gets to your house, it can open itself up, dump its contents everywhere, and make a self-replicating mess of your entire home…and then your neighbor’s homes, and the neighborhood, and so on. Let’s hope Amazon doesn’t sell any items like this anytime soon!
You can read about the viral life cycle here, but for the purposes of classifying a virus as alive or not, we are most interested in whether it makes energy, whether it takes in nutrients from the environment, and whether it excretes waste. It clearly can reproduce, after all!
Based on this, can a virus sometimes be alive?
Are parasites alive?
In the general use of the term, a parasite is an organism that lives on or inside a host organism in order to extract energy at the host organism’s expense. However, as a specific biological classification, parasites are eukaryotic (nucleus-containing) organisms that also meet the above criteria.
Under these definitions, are viruses parasites? Why do these distinctions even matter?
Cepelewicz, Jordana. New Giant Viruses Further Blur the Definition of Life. Quanta Magazine. March 5, 2018. [HTML]
Pearson, Helen. ‘Virophage’ suggests viruses are alive. Nature 454, 677 (2008). https://doi.org/10.1038/454677a [HTML]
La Scola, B., Desnues, C., Pagnier, I. et al. The virophage as a unique parasite of the giant mimivirus. Nature 455, 100–104 (2008). https://doi.org/10.1038/nature07218 [HTML]