One of the ironies is that a very common side effect of standard issue chemotherapy is a marked and often permanent weakening of the immune system by altering a person's microbiome. Advances in oncology perhaps within our own lifetime will make today's treatments seem as embarrassing as medieval blood-letting.
It's embarrassing now, never mind later. We just don't often have better alternatives than to pump already sick people full of poison in the hope that the poison kills the cancer faster than the rest of the person. It is horrible, and has (in my opinion) only improved slightly in the last few decades.