To be more clear, the disease is there. Just as the Small-pox disease is there. One has to become diseased, being infected. Similarly, the forms of the body are there. One has to infect the form and it develops. karanam guna sango'sya sad-asad-yoni-janmasu. Because he is infecting different diseased forms, therefore he is developing different forms of body. There is no such case of Small-pox changing into Malaria. Small-pox is Small-pox, and Malaria is Malaria. By the desire of the living entity, he becomes infected with a certain disease or body. Krishna Consciousness is the vaccine for all the diseases (different forms of material bodies).
Letter to Dr. W.H. Wolf-Rottkay, 18 June, 1975