The complexity of the human body has fascinated medical science, and every new discovery leads to a new level of complexity. Now it is no longer possible to talk about depression, for example, as a general disorder or cancer as a single disease. It may be that the brain of each depressed patient is depressed in its own unique way, and the leading research on cancer is heading towards personalized drugs targeted to each patient's highly specific genetic variation of cancer.
Does increasing complexity actually clarify things? The traditional disease models taught when I was in medical school are fraying around the edges, and some disorders, such as schizophrenia, have no localized cause. There is no known cause for schizophrenia. The general public thinks that you catch a cold because of exposure to the cold virus. But in fact direct contact with the cold virus gives only a 1 in 8 chance of catching cold.