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"Scanning electron micrograph of human coronavirus (small spheres, 80-120 nanometres in diametre) absorbed to the surface of a cultured cell. Coronavirus, in humans, causes the common cold & gastroenteritis; similar disease patterns occur in other animals. Like all viruses, the coronavirus must infect a host cell in order to replicate. The nucleic acids contained in the head are injected into the host, where they instruct the host's genetic apparatus to synthesize the component parts of the virus. Once produced & reassembled, the virus bursts (lysis) out of the host cell destroying it in the process. Magnification X 22,000 (at 12x10cm size)." (KEYSTONE/SCIENCE PHOTO LIBRARY/DR STEVE PATTERSON/SCIENCE PHOTO LIBRARY.)