Asking whether or not viruses are alive strikes right at biology’s roots. It makes us wonder about what the meaning of “life” is to begin with, and without a strict definition for life, “biology” doesn’t mean anything. If it’s the study of life, but we don’t really have a strong idea of what life is, then what the heck are we studying?

So just what IS “life”?

As a writer, I could define life as a summer’s day, or a baby’s smile, or that crowning moment right before your first kiss with the love of your short time on this Earth, but as a scientist I have to decide on a strict material definition. Not because I need to remove the romance from life, but because I need terminology that allows me to communicate my findings with others in a way that they’ll understand and be able to check in their own labs.

So, there’s a classical definition of life that we use. In biology, life is considered to be anything that eats, outputs waste, grows, adapts to new situations, and can reproduce itself as a recognizable entity that will then share those traits. That definition is meant to apply at a species level; biologists are not trying to suggest that infertility means you aren’t alive.

Under this definition, though, it’s classically taught that viruses are not alive. This is based on how viruses work, which isn’t usually taught all that well. If I had a nickel for every journalist who’s confused viruses and bacteria, I could singlehandedly fund the lab where I’m doing my PhD. And maybe retire to the Caribbean.

What Makes Viruses so Different?

All viruses have three basic components: outer structure,internal structure, and nucleic acid genome. Now, cells also have internal structure, external structure, and genomes. But there are key differences between the internals of a cell and those of a virus. Cells contain elaborate factories for turning food into energy and factories that spend that energy to build machinery that allows the cell to survive and reproduce. Viruses don’t contain these factories. They cannot eat. They do not make their own internal and external machinery. To do these things, a virus must infect a cell and hijack its factories, using the host resources. For this reason, we call viruses obligate parasites, meaning they cannot even reproduce without a living host.

Contrast this with bacteria: bacteria are themselves cells, and they contain all the factories that they need to eat, digest, and reproduce. They may sometimes parasitize other living things and eat them, but they can still reproduce and eat without infecting another organism. A bacteria that can infect you and eat your cells while you’re alive can still infect you when you’re dead and have no problems munching away. A virus can’t survive in a corpse; it needs its host to be alive to supply all that crucial factory apparatus.

If that’s the case, then viruses aren’t really alive.

If our definition of a life is something that eats, digests, grows, reacts, adapts, and reproduces, and a virus can’t eat, digest, grow, or reproduce on its own…we have to say that viruses aren’t “life,” at least in traditional biology. We could get around this pretty easily just by using a different definition for life. A good one that I’ve heard is to say that life is anything which is subject to natural selection. That’s a decent way to re-characterize the problem—and I also think it provides a lot of meaningful insight based on modern biology. Still, we should consider alternatives to just changing up the definition.

But Viruses DO “Live”—by Infecting Hosts

To do this, let’s examine why viruses, which wouldn’t be “alive” by typical reckoning, cause a problem for the traditional definition of life.

The main problem is that when viruses infect a host, the virus takes over host cells and uses them to eat, digest, grow, and reproduce on behalf of the virus. The host cells become completely filled with viral products, practically killing themselves — and in many cases actually killing themselves — to make new virus particles. In other words, once a virus infects its host, it can hijack the host’s cells and use them to do everything we’d think of as “life.” If that’s true, then it’s not so clear cut whether or not viruses are alive.

There’s another kind of biochemical “thing” we know about that needs to encounter another cell in order to grow and reproduce, though: male gametes, or sperm. Sperm can’t grow on their own. Their whole aim in life is to fertilize an egg, or to help their brother sperm achieve that goal. If they don’t, they die. But we don’t argue whether or not sperm are “alive;” they’re part of a living organism and produced by that organism’s life pattern. Asking if a sperm cell is alive is like asking if your toe is alive. Indeed they are not alive in isolation, but in the context of the reproductive system they are a crucial part of the organism and its life cycle.

In that case, the difference we imply between viruses and sperm cells is that sperm cells came out of a body. But viruses also come out of an organism, don’t they?

The problem is, we focus on the virus particle. The virus particle is that tiny package of external structure, internal structure, and genome. Some have called it a chemical with the potential to self-replicate. I don’t believe that this takes the full picture into account, though.

Viruses take over their host cells

When we talk about a given virus, we do use the term “life cycle.” Below, you see the life cycle for influenza virus, which causes the flu. Don’t focus too much on the details, just take it in:

Instead of the details, the key thing to take away from this image is the fact that most of the diagram takes place inside the cell. When the virus is just a particle, it can’t do all this stuff — all of which is critical to the virus copying itself and getting out of the host.

In the meanwhile, virus infection radically changes the cell. Viral machinery — proteins that either come in with the virus or are produced later — takes over the cell machinery. The cell becomes subverted to the virus and completely filled with viral products.

This represents a total departure from normal cell operations, and thus we should view the infected cell as entirely different from the uninflected one. As a result, there are those who consider the infected cell to be a distinct kind of viral entity; the result of the viral particle unpacking itself and becoming a living thing—sort of like how a seed becomes a tree. If that’s the case, then perhaps it’s the virus infection, but not the virus particle, that we can think of as “alive.”

Viruses can become sophisticated and specialized

Extending this a bit, many viruses produce systemic effects on an entire organism. There’s evidence that viruses do this through collaboration between various types of infected cells, and perhaps even through a high mutating to produce “specialized” particles with specific effects; a swarm of individuals that work together to achieve transmission to the next host.

In other words, virus infections produce particles that don’t even necessarily transmit to the next host — instead, these particles exist to support the overall infection. By doing that, these “suicide” particles encourage the few other virus particles that ultimately spread the infection to its next host.

Compare this with a complex, multicellular organism like a human. We all have specialized cells, and some of those cells exist for just a short time and then they die. Red blood cells, which are already pretty much dead by the time they’re flowing through the blood stream, are a good example. That level of specialization may also be used by viruses, and that makes the idea I’ve already presented — that the virus infection, not the virus particle, is a living thing — a compelling, useful thought experiment.

By forcing ourselves to confront the “virus life question” within the traditional definition, we’ve come to a new understanding of viruses, one that places more emphasis on the virus in the context of its host rather than the virus in isolation. Considering that “Virus-Host Interactions” is a key subfield of virology, I think this is a pretty important perspective to gain.

Instead of shackling us to a definition of viruses as inanimate chemical entities, by sticking to the initial definition we can change our perspective. We went into this thinking of the definition as an arbitrary construct meant to guide our inquiry, and by sticking to that construct we arrived at a destination where our inquiry has given us a new perspective. If we’d just thrown away the definition, we wouldn’t have been challenged into thinking, and if you don’t force yourself to think…how can you do science?

This article first appeared on www.johnskylar.com, but has been updated and substantively edited from that original form, which can be found here: http://www.johnskylar.com/post/79978994563/virus-life-for-me