Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Alzheimer disease ?
    ### Response:
    Alzheimer disease (AD) is a degenerative disease of the brain that causes gradual loss of memory, judgment, and the ability to function socially. Alzheimer disease currently affects about 5 million people. About 75 percent of Alzheimer disease cases are classified as sporadic, which means they occur in people with no history of the disorder in their family.
    