Book excerpt: Viruses are the last frontier of undiscovered life on our planet. The most abundant type of organism on Earth, they infect all types of cellular life, and, as micro-organisms that cause disease in their hosts, they are highly opportunistic and relentlessly efficient. They exist at the vanguard of DNA variance, exhibiting more structural diversity than plants, animals, archaea, or even bacteria. This 21st-century guide offers an engaging introductory section explaining exactly what viruses are and how they operate, followed by individual profiles of 101 of the world's most notable examples, each with its own dazzling mugshot