when a virus has infected a host, replicated, passed to a secondary host, then symptoms appear, then X occurs,
You have a situation where X has no influence on reproduction, replication or transmission, it just is not even on the same blackboard as that equation.
here is some suggested reading for you, not everyone understands evolution or natural selection very well so it is a bit of a TLDR
It is true that a basic law of reality/nature is that causes must precede effects, so in a sense, I think I agree your statement is technically correct regarding the exact cycle you describe.
But when talking about evolution in a population, the cycle happens a huge number of times. So "X" will influence all of the instances of replication that in fact happen after it. It would be silly to talk about evolution in a context of one and only one reproduction event.
I don't think we are debating anything that has to do with specialist knowledge of viruses.
So, specifically in answer to your question, it seems obvious to me that "lethal characteristics" are context dependent, so they come from random chance when an organism (or virus) encounters a new environment that it can replicate in way easier.