Rabbit Fur and Spotty Color Change?

I have a (aprox.) 2 year old French Lop that I rescued from an abusive home - I've had him for almost 6 months now. I take him to the vet regularly... in fact, I was there two weeks ago and the vet said he was normal and healthy. I know that rabbits can shed and have color changes due to weather and such, but he is not undergoing total body color change... it's only on his nose. There is one patch on his snout that has increasingly gotten darker and it's "spreading". I called the vet who reported back to me that this is a tell-tale sign of a vrius if accompanied with attitude/behavior change. (He has had no change in attitude/behavior, and if I poke the area, he is not in any pain.) I'm bringing him back to the vet in a week.... but was wondering what type of virus causes this and/or what can be done to treat it? I've searched the internet and tons of books, and I've found nothing similar to Mr. Bojangles' situation. Any help would be greatly appreciated.