I believe that there are different types of diseases that affect different type of organs. I'll just give an example. An autoimmune disease is a type of disease that includes it taking the form of one's cells in order for it to "camouflage" which will cause an easy opportunity for it to penetrate the defense system, or the immune system. Diseases come in through air, water, food. Some are contagious while others just come in through one's self action.