Any of the viruses that infect fungi (mycoviruses). In general these viruses are spheres of 30–45-nanometer diameter composed of multiple units of a single protein arranged in an icosahedral structure enclosing a genome of segmented double-stranded ribonucleic acid (dsRNA). Viruses are found in most species of fungi, where they usually multiply without apparent harm to the host. Most fungal viruses are confined to closely related species in which they are transmitted only through sexual or asexual spores to progeny or by fusion of fungal hyphae (filamentous cells). Some fungal strains are infected with multiple virus species.