Swine flu (H1N1) virus particle. Computer illustration of a swine influenza (flu) virus particle. At the core of the virus is RNA (ribonucleic acid) genetic material. This is surrounded by a nucleocapsid (green) and a lipid envelope (pink). In the envelope are two types of protein spike, haemagglutinin (yellow) and neuraminidase (red), which determine the strain of virus. Haemagglutinin allows the virus to bind to and enter a host cell, where the virus uses the cell's machinery to create more copies of the virus. Neuraminidase allows the new virus particles to exit the cell, so that they can infect others.
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