Mice (IMAGE) University of Utah Caption Two mice face off. A new University of Utah study found that a viral infection becomes more severe or virulent when it is passed from mouse-to-mouse and the mice are genetically identical. But when the mice come from different breeds -- even within the same species -- the virus loses virulence as it spreads from one mouse to another. Credit Wayne Potts, University of Utah. Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.