They don't breathe, they don't eat, they don't drink and they don't sleep. They don’t feel joy or pain. In short: they aren’t real life-forms. The first virus was identified in 1892, by Dmitri Ivanovsky: the Tobacco Mosaic virus. A few million different viruses have been discovered since then, although only 5,000 of these have been fully characterised.
Viruses have been found in the planet’s ecosystems: from the ocean floor to the tops of the highest mountains. If we take a litre of sea water, we can find 25 different types of viruses. In a kilogramme of marine sediment, there are around a million. This is possible because their sizes range from about 20nm in diameter (20 billionths of a metre) to a maximum of 250-300nm (the diameter of a human hair is about 300 times this!). Zoom out: it is estimated that there are about ten thousand billion, billion, billion individual viral units on Earth: 10³¹.