Below is an instruction from Human. Write a response.
    ### Instruction:
    How many people are affected by sickle cell disease ?
    ### Response:
    Sickle cell disease affects millions of people worldwide. It is most common among people whose ancestors come from Africa; Mediterranean countries such as Greece, Turkey, and Italy; the Arabian Peninsula; India; and Spanish-speaking regions in South America, Central America, and parts of the Caribbean.
    