Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Vasculitis ?
    ### Response:
    Vasculitis is an inflammation of the blood vessels. It happens when the body's immune system attacks the blood vessel by mistake. It can happen because of an infection, a medicine, or another disease. The cause is often unknown.    Vasculitis can affect arteries, veins and capillaries.
    