This is a book about how man discovered the world of super-small creatures. About the viruses that cause smallpox and rabies, influenza and polio, yellow fever and foot-and-mouth disease. About scientists who for decades fought to see and neutralize these invisibles. About how a virus that fraudulently enters a cell sometimes becomes an assistant to a scientist who reveals the secret of life. What is a virus - a creature or a substance? In the center of the book is the image of the wonderful Russian scientist D.I. Ivanovsky, the discoverer of viruses.