The influenza virus has three genera: A, B, and C. All can infect humans, but only A is responsible for illness on the pandemic scale. So it’s A that attracts attention. The virus comes in many different guises. The version currently besetting us—swine flu—is more properly called H1N1 2009, the letters referring to two all important glycoproteins dotted over the surface of the viral envelope. H stands for haemagglutinin: a molecule that anchors the virus to any cell it seeks to enter. No anchorage, no entry. Given the key role played by haemagglutinin, it’s no surprise that this is the antigen used to prepare antiflu vaccines. And then there’s N, short for neuraminidase. Accounting for up to a quarter of the viral surface protein, this is an enzyme that helps invading viruses to digest their way through mucous secretions as they approach the host cell, and later it assists in the release of newly synthesised virus. Neuraminidase too is tactically important to medicine, because the antiviral drugs oseltamivir (Tamiflu) and zanamivir (Relenza), work by inhibiting it.