Influenza virus particles, coloured transmission electron micrograph (TEM). This virus is the cause of the respiratory disease influenza (flu) in humans and other animals. The virus consists of a ribonucleic acid (RNA) genome, surrounded by a nucleocapsid and a lipid envelope In the envelope are two types of protein spike, haemagglutinin (H) and neuraminidase (N), which determine the strain of virus.