Purusa means the living entity, or the soul. He is subjected to be infected by the modes of material nature. There are 3 modes of material nature, sattva-guna, raja-guna, tama-guna. And when they are mixed up, they become 3 x 3 = 9, and 9 x 9 = 81, and in each quality, there are millions of forms. Therefore the total number of forms is 8,400,000.
So, the diseases are there and the forms of disease are also there. Material form means diseased form. Therefore, one has to change this form - death. And he carries with him the infection and he develops another disease. In this way, the soul although part and parcel of God in diseased condition, they are forced to change body. That is transmigration of the soul from one body to another.
Letter to Dr. W.H. Wolf-Rottkay, 18 June, 1975