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 concernin…
The plague is not actually that easily transmitted. Basic social hygiene prevents it. It is still around and causes very nearly zero problem today.