Viruses are the most common biological entities on Earth. Experts estimate there are around 10,000,000,000,000,000,000,000,000,000,000 of them, and if they were all lined up they would stretch from one side of the galaxy to the other.
You can think of them as nature’s own nanotechnology: molecular machines with sizes on the nanometre scale equipped to invade the cells of other organisms and hijack them to reproduce themselves. While the great majority are harmless to humans, some can make you sick and some can even be deadly.
Watch the video of how Viruses spread below.