[headmelted]

Why 20%?

That also seems wildly optimistic. 80% seems like a more reasonable assumption than 20%.

Also - 99.9% of those patients (pulled out of a hat) wouldn't have access to health care because the capacity was already overwhelmed, so the death rate will jump markedly.

[kragen]

Because 712 out of the 3711 passengers and crew were infected, and 713÷3713 ≈ 19.2%. So that gives us some sort of vague idea how much of the population from which the ship was drawn will become infected if exposed.