Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Flu ?
    ### Response:
    Flu is a respiratory infection caused by a number of viruses. The viruses pass through the air and enter your body through your nose or mouth. Between 5% and 20% of people in the U.S. get the flu each year. The flu can be serious or even deadly for elderly people, newborn babies, and people with certain chronic illnesses.
    