Influenza A virus particles, coloured scanning electron micrograph (SEM). Each particle consists of a core of RNA (ribonucleic acid) genetic material surrounded by a protein capsid, and a lipid envelope. Embedded in the capsid are two types of glycoprotein spike, haemagglutinin (H) and neuraminidase (N), which determine the strain of virus. Influenza A is the most virulent human pathogen of the three influenza types. Magnification: x41,600 when printed at 10 centimetres wide.