This beautifully written book introduces the origin and evolution of viruses, their significant influence on biological evolution and the development of human civilizations, and human beings’ struggle against viruses. It traces the love–hate relationship between viruses and life on the Earth across hundreds of millions of years from an evolutionist perspective: on the one hand, viruses, as cold-blooded killers created by nature, bring disease and death; on the other hand, as vectors of the horizontal transfer of genetic materials, viruses have played an inestimable role in the evolution of life, including the evolution of human beings. Viruses are both the deadliest enemy and the most valuable ally of human beings. Human beings must learn to coexist with viruses in order to survive.