ELI5: Why is there so much debate in virus taxonomy?

This mess starts with defining a species. That's a very messy process to begin with. We have around a half-dozen definitions of a species because none will work for everything. For instance, the most commonly used definition of a species is two animals that can mate and produce fertile offspring. But that definition can never include things that reproduce asexually. So we need more definitions for them. Then we have species tat sometimes hybridize and sometimes produce fertile offspring. So where do we put those?

Then we run into another definition problem. How do we define life? Again we have a lot of definitions but they will generally include things like : growth, reproduction, functional activity, and continual change ending with death.

So now what's a virus?
In its simplest form a virus is basically genetic material encased in proteins. It attaches to a cell and injects its genetic material which tells the cell's own machinery to create new viral genetic information, as well as new viral proteins. Those things come together in massive quantities and then they become new virus particles. The cell bursts and the new virus particles go onto infect new cells. But the virus particle retains the shape of the virus that originally infected the cell.

So with our definition of life: They don't grow, they can't reproduce without a host cell's machinery, they are functionally active but they don't change and they don't die in the same sense as other life, they just kind of fall apart.

So viruses don't fit our definition of life. They also don't fit any of our definitions of species.

So biologists can't even decide if they're alive. It's literally something that's debated.

Then there are about 4 different systems to classify them and those systems classify based on things like what organisms they infect, what kind of genetic material they use (DNA or RNA), their shape, things like that. But that's not how biologists classify organisms. We classify organisms based on their evolutionary history.

So we can't decide if they're alive. They fit no species definition. There are a multiple classification systems. And none of this comes together to add up to nice clean organization.

So people fight about it.

/r/explainlikeimfive Thread