Diseases begin to develop as more and more oxidative stress accumulates in one organ system.
When a critical amount of damage occurs, we can identify an actual disease, such as diabetes, atherosclerosis, stroke, cancer, etc.
These diseases just mentioned are all "diseases of aging".
Some diseases are the result of simple insult to the organism, such as infections or injury from predators or trauma.
Infections and predation are not the result of aging, but involve the risks of living in one’s environment.
As the average lifespan of our species has increased, we have seen the appearance of the diseases of aging.
These particular diseases occur in the older members of our population and are really not seen with any frequency in younger persons.
But, as our average lifespan increased, we began to develop these diseases, all of which are caused by free radical damage.
Probably more than any other disease, cancer is a disease of aging.
Old lab animals, such as mice and rats, in very controlled environments, mostly all die with cancer.