Flu virus particle structure, illustration. The internal core of the virus particle is shown here as segments of ribonucleic acid (RNA) in the viral genome. This core is encased by a protein coat (the capsid) and then by an envelope of lipids (here labelled the envelope, orange). Embedded in the envelope are spike proteins, with two types shown here that are specific to the influenza (flu) virus, such as the H1N1 subtype. The H-spike is the glyoprotein haemagglutinin. The N-spike is the neuraminidase enzyme. These protein spikes bind to the host cell and play a vital role in the life cycle of the virus. Viruses are very small, as shown by the scale bar (50 nanometres) at lower right.