This article points out, that viruses, in an interplay with RNA interference and as vehicles for intergenic and interspecies gene transfer, may work as agents for intracellular gene modulation, for steering of individual morphogenesis and as a driving force of evolution in the toolbox of nature. This is illustrated in particular in the light of a fungal double-stranded RNA virus that may be employed as a suitable agent for a biological control of aflatoxins, the most carcinogenic natural substances occurring in food and feedstuff.