Could be a virus, parasite, bacteria, etc...could also be due to chemical or heavy metal exposure, etc...could also be physical or emotionally stress induced. You need to remove the antigen(s) whether that be a biological, emotional, or chemically induced.
As for cancer, I do see it as an autoimmune disease. The TH1 system has tanked in most cases and the TH2 system can't keep up with an accelerated growing mass, circulating tumor cells, or cancerous stem cells.
I think the body is miraculous at healing however when we give it a crappy environment, how can we expect for it to work properly? Unfortunately all the chemicals, processed foods, and GMOs we are exposing ourselves to make our bodies look like an episode of hoarders. Unless we detox and remove the junk, only then will the good things we put in it be able to make changes.