All cells (bacteria, as well as body cells) function from instructions from their DNA. The (master copy of) DNA gets copied to (working copies of) RNA (single strands of instructions) and then the cell's proteins execute the instructions.

Viruses are NOT alive because they do not have all of the internal processes and organelles that cells have. They just have RNA protected by a sheath. If a cell takes in a virus, the cell's instructions will get corrupted.

The definition of "alive" is kinda fuzzy, but we're pretty sure that viruses aren't actually alive. It's probably easier to think of them like little machines that break into cells and convert them into virus factories. You can "kill" a virus by breaking it's outer shell, much like your phone would die if I snapped it's battery in half.

some viruses are DNA based, some are RNA based

A living organism is made up of cells that can replicate based on their DNA/RNA, which is the instructions used to perform the replication using the tools built into the cell structure.

Viruses are rogue instructions that repurpose an existing cell type to build something different.

Do we have any theories on why they might actually exist?

AFAIK, it’s roughly kinda the same theory we have as to why/how life itself exists. Certain chemical compounds—in the right structure—could potentially spontaneously allow for reproduction, based on the physics/chemistry of those structures. Some of those eventually evolved into what we now are / know as “life”, while viruses are (in some ways) perhaps a more primitive version of that.

It's not about whether they exist or not. They definitely exist.

It's about "what is life?" which is a much, much harder question to answer.

The truth is, it's a separate question. It's easy to define what a virus is, but you can debate all day about what exactly is life, and whether viruses fit into the definition that you land on.

If you're asking what a virus is, it is genetic material, wrapped in protein, that infects other cells, usually very specific cells, repurposing the cell to create copies of the genetic material, wrapped in protein.

Imagine that you open your letter box, and inside there’s a crumpled-up paper ball and a letter addressed to you.

You find the paper ball suspicious, so you dump it, and you take the letter in. You open the letter, and its from your aunt, who is one of those chain letter types, and she’s sending this nice, handwritten postcard asking you to make 2 more such postcards and send it to people you love and care about.

Since it’s from your aunt, and you figure why not, after all the effort she apparently went through to put it in an envelope and all, and now you’ve made 2 more such postcards and sent them along to your friends/relatives/neighbors.

The letter in this case is like a virus because it has the instructions needed to make more of itself (the genetic material) and a packaging that it comes in (the envelope) that allows it to enter your house (your cells).

It’s not alive in the sense that it can’t make more of itself, by itself. The letter can’t make more letters without you (a cell), your ability to read the instructions (the enzymes that can replicate the genetic material and read it), and the printer/ink/paper in order to actually make the new letters (your ribosomes and DNA or RNA polymerases, depending on the kind of genetic material in the virus).

Now realistically, if the letter were to function like an actual virus, it might have instructions instead to "Print more copies until your printer catches on fire and explodes" and "Mail the letters to every person you personally know", and after this letter gets around for maybe a day or two, the law enforcement shows up to bust your chain letter syndicate by burning your house and all affected houses down (which is kinda analogous to the programmed cell death process of apoptosis), assuming your house wasn't already destroyed by that aforementioned printer. But that's a discussion for another day.

Viruses are DNA or RNA in a capsid (protein) shell

They are not considered "alive" because they do not do anything that a living thing requires. No organs (organelles in microbes), do not carry out any metabolic processes (no eating, growing, use energy, etc), and the biggest thing in microbiology class they do not reproduce without needing a host.

The reason the non reproducing thing is the biggest point is because a lot of other microbes are kind of similar to viruses in the sense of: very little to no organelles (usually have ribosomes), little to no metabolic processes at points in its life cycles. But all microbes can reproduce without a host (such as through binary fission). Viruses need a cell to act as its baby factory, hijack it's organelles (ribosomes usually) to make copies of its DNA or RNA

Side notes: ribosomes are organelles (organs) that take rna (instructions) and make protein (building block of life) (making protein is called protein synthesis, this is called the central dogma of biology). Viruses can use these to make new viruses (since they can't make protein themselves).

A lot of simple bacteria have only dna and ribosomes (so little to almost no organelles)

Binary fission is what you typically think of when cells split to make 2 new cells

DNA is double helix, RNA is a single strand, they get their different names based on their sugar base on the strands. Ribonucleic acid (ribose sugar) deoxyribonucleic acid (derived from ribose but lacks a oxygen (hydroxyl) in it). DNA is basically the "master instructions" to make a cell/living thing, RNA is a more compressed version of it (basically a smaller copy).

They didn't question whether they exist, they asked why.

When a cell dies, it usually releases its genetic material into the environment. Wild organisms are constantly running across DNA/RNA from dead cells. Some organisms have found that picking up random DNA can be beneficial as it can lead to obtaining genes that will give the organism a special advantage. Imagine this happening millions and millions of times over a period a day. It's not hard to see how some stands of DNA/RNA that make it into cells cause adverse effects and eventually develop into more complex items

Ah a pointless tiny hill I’m willing to die on.

The only reason viruses are called not alive is because of our elitist view that alive things have to do anything other than self replicate.

A tiger has its claws and speed and muscles to do one thing. To make another tiger. It uses its assets to get food and mate and makes another tiger and incidentally during that time “lives” by doing other stuff that does not involve making tigers.

Now if you condensed being alive to its purest form. Making more copies of yourself. And then you went even further. Ditch any self replicating mechanism, just have a way to take over someone else’s. You get a virus.

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