Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) West Nile Virus ?
    ### Response:
    West Nile virus (WNV) is an infectious disease that first appeared in the United States in 1999. Infected mosquitoes spread the virus that causes it. People who get WNV usually have no symptoms or mild symptoms. The symptoms include a fever, headache, body aches, skin rash, and swollen lymph glands.
    