Issels believed that cancer was caused by the weakening of the human immune system and hence had to be cured by strengthening it again. However, he did not dispute the importance of conventional cancer therapies like surgery and chemotherapy, and did in fact use them when treating his patients. Issels did not advocate a panacea-like new therapy, but rather prescribed various neglected, forgotten, or non-mainstream treatments, such as the Coley Vaccine pioneered by William Coley, hyperthermia, where Manfred von Ardenne researched its effectiveness in cancer.