ELI5: How a virus isn't really alive.

Scientwist · 4 months

Some of these answers are partially correct but I'd like to expand on them a little bit:

It all boils down to "How do we define life?" That's a tricky question, right? It seems obvious on the macroscopic level but once you go microscopic things get more muddled.

The most universal definition of life is not a true definition! Instead it is a checklist:

Does the object undergo homeostasis (that's a big word for maintenance)?
Does it metabolize chemicals to create energy for itself to utilize?
Does it grow and adapt to changes in it's environment?
Does it reproduce to create new objects?

If the answer to all of the above questions is yes, then (scientifically speaking) the object is alive.

Now to get to viruses. You probably know (from other responses or previous teachings) that a virus hi-jacks a host cell for it's own benefit. But I doubt you've ever been told why. Viruses do this because they are unable to reproduce (which they do in a unique and very complicated manner) on their own. They need to infect a different cell in order to reproduce.

If we follow the strict definition of life (the list above) then it appears that viruses fail to meet this requirement. This is where the canonical "viruses aren't alive" statement has it's roots. Without a cell to infect, the virus will eventually die off. Your point is well taken, however. This is all a semantic argument over definitions to non-biologists.

An aside; a few years back there was this huge discovery about viruses that has since led people to refute the claim that viruses aren't true lifeforms. I can go into detail about that if you'd like but hopefully this post answered your original question.

RussellG2000 · 4 months

Life means you can get energy from eating something (plants, animals, sunlight, minerals, and so on), you can have babies of your own that grow up and have more babies, and you can adapt to your surroundings to survive. A virus on the other hand does not eat anything else to make energy to live. There is still a lot of debate over whether or not a virus is alive, but for now it isnt.

ameoba · 4 months

When people were first discovering how viruses worked, a lot of scientists felt the same way as you. There was a lot of argument of what 'life' really was. In the end the finally decided that "life" needed to reproduce on its own and that was that.

tadrinth · 4 months

Metaquestion: Why is there confusion and disagreement about the definition of life and whether or not a virus fits that definition?

Living things have a number of properties in common, because all life has a common origin and has been strongly optimized for the ability to replicate.

Since living things have reliable properties and were important to the reproductive success of your ancestors, those who had an instinctive, hardcoded intuition for living things were more reproductively successful. Since living things have a lot of properties that aren't found in things like rocks or tools, the intuition is complicated (lots of factors involved).

So, modern humans have a very strong and very complicated intuition about life. However, that intuitition developed to recognize the living things our ancestors dealt with, not viruses. Evolution doesn't optimize for what it doesn't see. Your intuition delivers a very sharp signal for everything a cave man ever saw, but it doesn't deal so well with viruses.

So, when people try to define the word "life" what they actually do is try to convert their complex, multifactored intuition with fuzzy boundaries into a simple set of rules with sharp boundaries. You can imagine how well that usually works. Ideally, you have a set of rules which closely enough resembles the intuition that when someone hears your definition, they recognize it and map it onto their own intuition.

The problem with viruses is that they have some of the factors that our intuition maps onto 'life' but not all of them. That confuses the hell out of our intuition, so not everyone agrees as to their classification. No matter what definition you pick, someone will object that their intuition disagrees on edge cases like this, so there is no satisfying answer.

A much better option is to use the term 'replicator'. You can come up with a much simpler definition for it because it isn't trying to convey a massively complicated intuition. Replicators make more copies of themselves. You can get pretty close to a mathematical or technical description.

So, are viruses replicators? Yep. Sperm replicators? Yep.

Humans also tend to think of things in terms of a vital force (a mistake called vitalism) even when they should know better, because that's how our intuition is wired. Using 'replicator' instead of 'living' helps reduce that particular cognitive bias.

The best option is to be specific about what properties you care about, rather than using an unsatisfactory or confusing definitions.

2dollarb · 4 months

Viruses are more similar to machines than to life.

They do not eat.
They do not sleep.
They do not respirate (breathe).
They cannot think or feel.
They cannot reproduce without a host.

Viruses are like a special mechanical key. The key fits into a special lock on the outside of special cells. Once the key is inserted the mechanics start to work. The Virus then runs through its program combining the DNA or RNA with its host cell, creating more of the virus keys to infect more cells.

poopsmith1976 · 4 months

Viruses need a host cell to reproduce

miamiburn · 4 months

First off, you must understand the two most defining observable features of a virus - the protein coat, and the DNA within this coat. If they have DNA, one would assume they are alive though, right? Wrong. Viruses do not eat, sleep, breathe, reproduce (without a host), have any sense of their surroundings, or grow. Furthermore, they are non-cellular. Hopefully, this clarifies things a little bit.

Source: I'm a Biosystems Engineer.

hydroxy · 4 months

Living organisms undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations

Viruses can do none of these on their own, without a host cell they are just protein/nucleotide structures floating around doing nothing

blueskin · 4 months

They do not move autonomously, either consciously or reactively
They have no senses
They have no biological process such as respiration, and indeed, do not make use of energy or resources
They can not reproduce unaided

There is no strict definition of life, but a virus fails on the majority of commonly-accepted criteria.

iamvillainmo · 4 months

Viruses lack most of the machinery needed to replicate independently of a host, which are defining characteristics of life.

FWIW, many scientists argue that viruses should be considered to be life but they are really just assemblages of protein and nucleic acid (DNA or RNA) that are inert until they infect a susceptible cell.

reggionh · 4 months

i guess the reason why its hard to say that they're living beings is the fact that they dont have DNA. they have RNA instead.

but i could be wrong :)

Hindu_Wardrobe · 4 months

No cells, no life.

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