I would like to comment on some of the ideas in your post. First I am not an MD and most of my info comes from a series of presentations from professor Zajicek on you-tube.
Cancer is the result between a driving virus and host resistance.
" Also a defective immune system could have its ‘repair’ function working..." A strong immune system wipes out quickly the virus while a weak one tolerates a chronic inflammation. Inside the cell it is either the virus, which kills the cell and produce more virons (lysis) or its DNA is being incorporated into cell's DNA and protects it from similar viruses (Lysogeny).
"...allergy sufferers ever develop a cancer" If it causes a strong immune response, it certainly could result in reducing the virus load (lytic cycles / lysogenic cycles). You may want to look on Dr Coley work with Coly toxins, which may have cause a similar effect, which resulted in a long term remission.
"No single medicine has been discovered that works for everyone..." Please see the presentation "How to cure chronic Hepatitis" by professor Zajicek: LLV, discovered by Rubin in 1960 and blocks RSV, Rous sarcoma virus (RSV) which causes a chicken sarcoma.