JOHN BAVOSI / SCIENCE PHOTO LIBRARY JOHN BAVOSI / SCIENCE PHOTO LIBRARY
Flu virus. Cut-away illustration of the virus that causes influenza in humans. The red coil in the centre contains the virus's genes in the form of RNA (ribonucleic acid, similar to DNA). Around this is a protein coat, the nucleocapsid (white). This is enclosed in a membrane (or envelope) that is studded with complex spiky molecules called glycoproteins. These bind to mucous cells in the human respiratory tract, enabling the virus to in- fect its host. Flu viruses multiply inside mucous cells before being spread via airborne droplets produced by sneezing and coughing. They can evade the human immune system thanks to frequent mutations that change the glycoprotein envelope.
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