They are the most common creatures on earth. They have been around forever and are considered dangerous and deadly - but they are much more than that. Around 50 percent of our DNA comes from viruses. They help to form the immune system, give us long-term memory and make themselves useful in our digestive tract. But they can become dangerous when humans alter the natural habitats of animals and destroy biodiversity. This is what the history of epidemics teaches us, from the measles to AIDS to Ebola and now Covid-19. The film takes us into the fascinating world of virus research: Was the virus at the beginning of all life? Why can viruses be used to heal? Why do viruses suddenly become pathogens? How can pandemics like Covid-19 be prevented or at least effectively contained? The tiny protein particles in which hereditary information is packaged are much less a threat than indispensable architects of all life.