Very interesting question. People do still seem to misunderstand that creatures need to have an aim in mind when they do things.

The beauty of life was that it needs no more explanation than a ball rolling down a hill. https://twitter.com/MarkEglinton/status/1338621029334315013

You could say that the ball "wants" to go lower if possible. But ultimately it is being dragged that way by gravity.

We can use the term "want" loosely, when describing inanimate objects:

"The heat in this oven wants to spread out evenly"

But we know it is only shorthand

The mistake that leads people to think that viruses and other creatures particularly want to spread, is that Darwin's principle is often quoted as "Survival of the fittest", and we often misunderstand that as "Survival of the fittest creature".

In reality, Darwin's insight was,

"Whichever virus GENOME produces most copies of itself at present"

"... will (obviously) increase in prevalence the most, the next time you look."

Trouble is, when you put it like that, it's a bit face-palm-ily obvious.

The genius of life/Nature/God has been to get genetics working.

Once that is working in any useful way, God doesn't have to do anything.

Just wait a few billion years.

Lots of stuff will happen. The stuff that is most effective at creating more copies of itself will ... be the most effective at creating more copies of itself.

A few billion years later, the world is full of numerous genomes that have found various ways of duplicating themselves very effectively.

In almost all cases, the genomes do this by coding for a complete creature, which assemble themselves, parent-to-child.

But once there are enough of such genomes active, and therefore such creatures...

There is an opportunity for other genomes to replicate through parasitising on the creatures created by the other genomes. These are viruses.

Covid doesn't have a purpose, any more than a ball "wants" to roll down hill.

For shorthand, we say "it wants to spread"

But really we are stating the obvious.
The billions of Coronavirus particles that people in my hospital have are different individual particles from the ones wiping out people in China a year ago.

The deaths of the people are incidental. What the coronavirus genome is extremely good at doing, is duplicating itself into other people.

That is why we have such a big problem with it. Its extraordinary rate of spread in normal-behaving people.

Excellent point! https://twitter.com/ProfDConnelly/status/1338763370871336960?s=20

If a virus made lots of copies of itself, but only in one host, we wouldn't be talking about it.

Because it would just infect one person, and that would be the end of it.

Ever heard of Covid-18?

No?

Well, its because there was only one guy who had it. No world panic over that. Unlucky for him, but nobody else cares.

The reason we are all talking about Covid-19 is that it got itself into so many millions of people worldwide, so quickly.

The viruses that spread are the ones we care about.

The most successful virus would be the one which infects many different people, but with only one copy in each, so that the person survives to spread it.

However that is hard to do, and not worth the virus's while evolving a way to achieve.

Easier (i.e. more likely to be found by the billions of chance mutations) is to get multiplied in the respiratory tract, and then coughed, spoken or breathed into other hosts.

Whether the person dies or not doesn't matter a great deal to the success of viral GENOME replication. Rapid replication is an advantage because more particles make it easier to infect other hosts.

The "spare copies" of the viral genome that stay behind in the host don't contribute much either way. They are a bit of a waste, but not much of a waste, as the resources came from the host, not the virus.

The spare copies, that leave the poor host gasping for breath and perhaps dying, are no more of interest to the viral genome, than the shed skin to a snake.