A reduced level of virulence in a strain of pathogen resulting from genetic changes in the pathogen or to the effects of an infectious agent on the pathogen. 
Where a disease-causing organism is less able to cause the disease, or where the disease which results is milder. Decreased virulence in a pathogen. ... (09 Oct 1997) ... 
The phenomenon by which a strain of a pathogen is less virulent than normal. In some fungi infection with dsRNA is thought to be responsible.