The problem with cancer is that, much like with the HIV virus, the immune system is fooled into thinking the cancerous cells are part of the human body.

From what I hear researchers are optimistic that any such disease can soon (i.e. within the next 20 years) be cured by injecting blood-cell sized nanobots that can reliably target the foreign cells or viruses in your body and kill them (and, unlike chemotherapy, don't destroy the good parts, too). I think the chronic disease is just a crutch that'll soon be obsolete.