Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Polyarteritis nodosa ?
    ### Response:
    Polyarteritis nodosa is a serious blood vessel disease in which medium-sized arteries become swollen and damaged. It occurs when certain immune cells attack the affected arteries preventing vital oxygen and nourishment. Signs and symptoms may include fever, fatigue, weakness, loss of appetite, weight loss, muscle and joint aches, and abdominal pain.
    