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Illustration of the human coronavirus 0C43 (HCoV-OC43). Each virus consists of a core of RNA (ribonucleic acid, not seen) genetic material surrounded by a protein nucleocapsid and a lipid envelope (blue). The coronaviruses take their name from their crown (corona) of surface proteins, which are used to attach to and penetrate their host cells. Once inside the cells, the particles use the cells' machinery to make more copies of the virus. HCoV-OC43 is a cause of the common cold and can also cause pneumonia in infants, the elderly and immunocompromised patients. (KEYSTONE/SCIENCE PHOTO LIBRARY/SIMONE ALEXOWSKI / SCIENCE PHOTO LIBRARY)