Typically, a change in the host, not the antigen, is implied. Though some pathogens can evolve to become less virulent in host-pathogen coevolution, tolerance does not refer to the change in the pathogen but can be used to describe the changes in host physiology. Immune tolerance also does not usually refer to artificially induced immunosuppression by corticosteroids, lymphotoxic chemotherapy agents, sublethal irradiation, etc. Nor does it refer to other types of non-reactivity such as immunological paralysis. In the latter two cases, the host's physiology is handicapped but not fundamentally changed.