Many fungi, among which are some plant pathogens, are known to be infected by viruses. Infection by some viruses causes an attenuation of the virulence of certain plant pathogenic fungi, resulting in the existence of virulent virus-free strains and hypovirulent virus-infected strains of the same fungal species. Hypovirulent strains can convert virulent ones into hypovirulent ones, provided they are vegetatively compatible, by transmitting their viruses by hyphal anastomosis. Spontaneous natural control of chestnut blight, a disease caused by the ascomycete Cryphonectria parasitica, through virus transmission between fungal strains was first reported in Italian stands of Castanea sativa (European chestnut) in the mid-twentieth century. This finding led to the development of sucessful biological control methods of the disease based on the artificial introduction of hypovirulent strains of the pathogen in affected chestnut areas. As a consequence, the natural phenomenon of hypovirulence in the European chestnut-C. parasitica pathosystem has become a model for the study of similar ones with the aim of using fungal viruses as biological control agents against plant fungal diseases.