With up to two billion of them ineach litre of sea water, viruses play a vital role in controlling the marine environment and are essential to the ocean's delicate ecosystem. From their discovery and the unravelling of their intricate structures, Crawford demonstrates how these tiny parasites are by far the most abundant life forms on the planet. In this Very Short Introduction Dorothy Crawford demonstrates how clever these entities really are. From pandemics such as HIV, swine flu, and SARS, we are constantly being bombarded with information about new lethal infections.