For all our hubris, humans can be felled rapidly by an invisible enemy - viruses. All around us are minute entities that can damage and kill: the millions of viruses that pervade the natural world. Our bodies harbour many that we have long tolerated, but a new one, that jumps into humans from another species, can be lethal - as we have seen most recently with the virus responsible for COVID-19. But what are viruses, how do they cause disease, and how can we fight them? In Viruses: The Invisible Enemy, a brand new edition of her classic work, virologist Dorothy Crawford explores these questions. She takes the reader on a journey through the past to show how, as the human race evolved from hunter gatherer to farmer to our present urban, industrialised society, viruses have taken advantage of each lifestyle change to promote their own survival. We have acquired many new viruses along the way, some spreading globally and causing killer diseases. But now, in the 21st century, as humans increasingly encroach into and exploit the natural world, the rate of emergence of novel viruses is accelerating. Already we've had a flu pandemic, large epidemics from SARS, Ebola and Zika viruses, and most devastating of all, SARS-CoV-2, which swept around the world in 2020-21, causing the COVID-19 pandemic. The response of scientists has been rapid, producing vaccines in record time. But we can expect more such challenges in the future. This book discusses why and how SARS-CoV-2 and other killer viruses emerged, and how we can win the battle against such an enemy.