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Human coronavirus. Coloured transmission electron micrograph (TEM) of Human coronavirus particles (purple circles) in a host cell. Coronaviruses primarily infect the upper respiratory and gastrointestinal tract and can cause the common cold, gastrointestinal infections and SARS (severe acute respiratory syndrome). Coronaviruses are named after the corona (crown) of surface proteins (outer rings) that are used to penetrate a host cell. Once inside the cell, the virus particles (virions) use the cell's machinery to make more copies of themselves. (KEYSTONE/SCIENCE PHOTO LIBRARY/AMI IMAGES/SCIENCE PHOTO LIBRARY)