I am not a biology person, but, candidly, this is a bad article, because the author is generalizing from the properties of viruses that tend to arise naturally, and using this to argue about the properties of deliberately designed viruses.

Firstly, it's not even clear that naturally occuring viruses can't be extremely lethal. There are examples (eg black death).

Even the existence of rare examples should be concerning, if attempting to argue against the possibility.

But even if there were no natural examples, it could just be because the ecosystem is in a balance, which them unlikely. That doesn't preclude someone from designing one.

To use an analogy. Natural predator prey ecosystems tend to be in balance. You could look around the world, before modern humans, and conclude that superpredators aren't a real threat, because predators that eat their prey too rapidly typically then die out. Thus most ecosystems you observe are in some balanced state.

But this doesn't mean modern humans can't come along, using their superior intelligence, and act as a superpredator. Ask the wolf population of western Europe.

If something is stopping us from doing this more often, it's alignment rather than capability.

You can't generalized from the typical behaviour that you see in a balanced natural ecosystem, and use that to say it proves there are no much more dangerous possibilities.

If the author made first principles argument about the inherent difficulty of doing something bad, even with abundant intelligence, I'd be much more reassured; or about defense offense asymmetry favouring defence.

What's here seems very naive.

> Firstly, it's not even clear that naturally occuring viruses can't be extremely lethal. There are examples (eg black death).

You realize the black death didn't arrive and then go away again, right?

It was extremely lethal, once.