Viruses are big news. From pandemics such as HIV, swine flu, and SARS, we are constantly being bombarded with information about new lethal infections. In this Very Short Introduction Dorothy Crawford demonstrates how clever these entities really are. 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. With up to two billion of them in each litre of sea water, viruses play a vital role in controlling the marine environment and are essential to the ocean's delicate ecosystem. Analyzing the threat of emerging virus infections, Crawford recounts stories of renowned killer viruses such as Ebola and rabies as well as the less known bat-borne Nipah and Hendra viruses. Pinpointing wild animals as the source of the most recent pandemics, she discusses the reasons behind the present increase in potentially fatal infections, as well as evidence suggesting that long term viruses can eventually lead to cancer. By examining our lifestyle in the 21st century, Crawford looks to the future to ask whether we can ever live in harmony with viruses, and considers the ways in which we may need to adapt to prevent emerging viruses with devastating consequences.