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Illustration comparing the size of a red blood cell (erythrocyte, red), Escherichia coli bacterium (E. coli, purple) and a SARS-CoV-2 virus (far right, small purple and orange sphere). Their diameters from left to right are: erythrocyte (8500 nanometers), E. coli (1100 nanometers) and SARS-CoV-2 virus (140 nanometers). (KEYSTONE/SCIENCE PHOTO LIBRARY/MEDICAL GRAPHICS/MICHAEL HOFFMANN/SCIENCE PHOTO LIBRARY)