Alzheimer's disease
These small preclinical changes (no symptoms) in the brain may occur many years, to even a few decades before a person is diagnosed with Alzheimer's. With a stage where there is some memory loss, or mild cognitive impairment. These changes put a person at risk of developing the clinical symptoms of full-blown  Alzheimer's but not everyone who has these changes will get the disease. Even though there is no cure for Alzheimer's, there are new treatments which are being developed which would work better in the very first stages of the disease.