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> I thought the general theory of "virus evolution" was towards less severe variants.

That's probably only in the case where the disease is so severe it kills the host before it can spread very easily, not some general rule.

Let's say you have a virus that's super contagious for two weeks with no symptoms and nearly everyone who gets it dies within a month. What evolutionary pressure would it be under to change as long as it has fresh hosts to infect?