FAQs: H5N1 influenza. H5N1 is a type of influenza virus that causes a highly infectious, severe respiratory disease in birds called avian influenza (or bird flu).5N1 infection in humans can cause severe disease and has a high mortality rate. If the H5N1 virus were to change and become easily transmissible from person to person while retaining its capacity to cause severe disease, the consequences for public health could be very serious.