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Coronavirus particles (red) on the surface of a culture cell (yellow), coloured scanning electron micrograph (SEM). Coronaviruses are responsible for causing common colds and gastroenteritis. The virus particles (virions) enter a host cell and use its own cellular machinery to make more copies of the virus. These copies then burst out of the cell, killing it, before infecting other cells. Magnification: x18,300 when printed 10 centimetres wide. (KEYSTONE/SCIENCE PHOTO LIBRARY/DR STEVE PATTERSON)