Although viruses are almost always thought of as pathogens, most viruses probably do not cause disease, and some provide essential benefits to their hosts. Beneficial viruses are found in a wide variety of hosts including bacteria, insects, plants, fungi and other microbial eukaryotes, and humans and other animals. Beneficial viruses can confer tolerance to stress such as heat, cold and drought; they can prevent or attenuate infection by pathogenic microbes; they can act as bioweapons to allow their hosts access to new territory; and they have been critical in the evolution of their hosts. In spite of their bad reputation, viruses could be used to benefit humans and their food sources in novel ways.