evidence: The exact causes of Alzheimer's disease aren't fully understood. But at a basic level, brain proteins fail to function as usual. This disrupts the work of brain cells, also called neurons, and triggers a series of events. The neurons become damaged and lose connections to each other. They eventually die. Scientists believe that for most people, Alzheimer's disease is caused by a combination of genetic, lifestyle and environmental factors that affect the brain over time. In less than 1% of cases, Alzheimer's is caused by specific genetic changes that almost guarantee a person will develop the disease. In these cases, the disease usually begins in middle age.