Coronavirus are a type of virus which causes common colds, but can also cause more serious diseases such as SARS, MERS, or COVID - 19. The virus is only about 0.025 µm in diameter, far smaller than the human eye can see even with the best optical microscope. In comparison, a fine human hair is about 40 µm in diameter. When people talk, cough, or sneeze they eject small droplets which vary greatly in size but the average is about 1 µm. Each small droplets can hold hundreds of thousands of viruses and the droplets can persist in the air for several hours.

This image of a coronavirus taken by an electron microscope shows small structures on the surface which look like crowns. When the virus encounters a human cell, the crown attaches to the cell's surface and injects its own RNA into the cell, which then takes over the cell mechanisms and produces copies of the virus. They eventually cause the cell to burst which can release up to 50,000 new viruses.