How do Christian Scientists respond to the jellyfish argument?

Christian Scientists believe that illness is caused by a mental error, rather than a physical abnormality. In other words, that all diseases are psychosomatic. Critics say that, if that were true, animals without a brain (and thus without a mind) could never get ill. But jellyfish, animals without a brain, can and do get diseases such as cancer. How do Christian Scientists respond to that argument?