A comprehensive and richly illustrated introduction to the world of virusesAs parasites that are often hundreds of times smaller than bacteria, viruses exist in and on everything, everywhere. Rapidly evolving, they are highly opportunistic and relentlessly efficient. While some viruses are obviously agents of disease, as the COVID-19 pandemic has reminded the world only too well, others can be beneficial, helping to protect their hosts from other microbes, or allowing hosts to function in otherwise impossible ways.