causing illness in the host after transmission allows more contact between infected and putative hosts thus conspecific differential of reproductive success.
prospecific differential reproductive success occurs when variations are able to generate progeny that themlelves are capabe of reproducing. i.e. delta variant sars2
evolution occurs when the properties leading to these phenomenon are genetically based and transmitted through populations at varied frequencies subject to differential natural selection i.e. the fittest as in most apt to survive to reproductive modality.
Causing illness in the host after it has finished spreading has no evolutionary advantage.
So those traits are less likely to make it to the next epoch.