Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Stroke ?
    ### Response:
    A stroke occurs if the flow of oxygen-rich blood to a portion of the brain is blocked. Without oxygen, brain cells start to die after a few minutes. Sudden bleeding in the brain also can cause a stroke if it damages brain cells.
                
If brain cells die or are damaged because of a stroke, symptoms occur in the parts of the body that these brain cells control.
    