this is fact, this is evolution. when reproduction occurs this is success, this is fitness to reproduce. if a host is killed, or inconveinienced enough to lay ill and not contact others, that is a selective factor that favours reduction of morbidity. in the case of coronavirus reproduction and transmission occurs before morbidity occurs in the index host. there is no selective pressure to become less virulent. there is selective pressure to change host, to evade recognition by the immune system.