Cancer is a class of well identified diseases. Like a bacterial infection.

There are many types of cancer just like there are many types of bacterial infection. But they are both well identified. We can see the cancer cells and bacteria, and when they are gone, the patient is cured.

Mental illnesses are more like the "flu-like syndrome". The causes can range from leukemia to heroin withdrawal to a wide variety of viral infections like AIDS, the common cold, and of course, the flu. You can treat the symptoms with Tylenol but we can't really call that a disease, more like the symptoms of a yet undiscovered disease.