Plant-associated fungi are infected by viruses at the incidence rates from a few % to over 90%. Multiple viruses often coinfect fungal hosts, and occasionally alter their phenotypes, but most of the infections are asymptomatic. Phenotypic alterations are grouped into two types: harmful or beneficial to the host fungi. Harmful interactions between viruses and hosts include hypovirulence and/or debilitation that are documented in a number of phytopathogenic fungi, exemplified by the chestnut blight, white root rot, and rapeseed rot fungi. Beneficial interactions are observed in a limited number of plant endophytic and pathogenic fungi where heat tolerance and virulence are enhanced, respectively. Coinfections of fungi provided a platform for discoveries of interesting virus/virus interactions that include synergistic, as in the case for those in plants, and unique antagonistic and mutualistic interactions between unrelated RNA viruses. Also discussed here are coinfection-induced genome rearrangements and frequently observed coinfections by the simplest positive-strand RNA virus, the mitoviruses.