What are the illnesses that they give out ganja for?

Cancer. The fact that you call it "ganja" and ask which illnesses "qualify" for a prescription leads me to wonder if you may be looking for a reason to justify a behavior that you are already doing or wanting to do, i.e. Smoke marijuana. The legitimate medical uses for marijuana are very limited, e.g. To counteract the serious side effects of chemotherapy for cancer patients. I hope this answer helped.