Perhaps classical music calms the immune system, decreasing its responsiveness – the Traviata mice had fewer white blood cells and immune-signaling molecules. Or maybe mice just prefer Verdi to Enya.
I agree that it is the calming effect of some music. The last opera I went to was Salome. All I can remember is a man's head on a tray, not a calming influence at all. La Traviata starts out with a famed courtesan throwing a lavish party to celebrate her recovery from an illness. That's a far happier scenario for these obviously discerning mice than thinking about a severed head on a tray. These mice obviously know their opera.
Just think how the mice might have reacted to Elvis' song, "Suspicious Minds" (lyrics include "We're Caught In a Trap". That would have blown their white blood cell count off the charts.
