Recently, I found my immune system wiped out again.
I fell asleep and slipped into a state where my astral form was in control. I was confronted with what appeared to be some horrible demon trying to attack me. I recognised this as the source of some infection, and immediately started fighting it, eventually performing the Lesser Banishing Ritual of The Pentagram to throw it off.
In modern times, many of those who adhere purely to the "doctrines" of modern science would laugh at a shaman who travels into the spirit world to talk to and intercess with the spirits causing the disease, on behalf of the patient. Such "primitive" approaches are no longer valid they would argue, modern science can show that the viruses and other infections driving the disease can be clearly shown through various tests, without having to rely on the idea of spirits and such nonsense.
Ok, so the shamans may not have the scientific knowledge to understand the process of infection the way modern science does, but both modern science and shamans know there is some "entity" (in the loosest sense of the word) behind infection and disease, whether you see it to be a spirit or cell(s) that are causing the disease or infection.
There are people who have seen shamans who have recovered.
There are people who have undergone chemotherapy who have recovered.
Take all options available.
All that matters are the end results.