I had the grey carefresh for a while and Giz (I had her by herself for a few weeks) sneezed like mad. Other than the sneezing, nothing was wrong with her, no discharge, eating, drinking, playing fine, etc. I switched her to the Carefresh ultra to see if it would make a difference and voila, no more sneezing! Now I have 3 rats in a small-medium cage and I end up cleaning it AT LEAST every 3-4 days though. But I do like the carefresh ultra. by the way, I'm looking into getting them a bigger cage.