I honestly haven't the slightest where he got the infection from, but from examining his eye I would guess that it was from a scratch of some sort. Either debris or from his cagemate's claws. He had the infection when I got him, and the previous owner wasn't sure how to go about treating him. I happened to have aquired a chinchilla with a similar problem not too long ago, so had some antibacterial drops already on hand which worked very well. He is quite tame, so medicating him wasn't a real problem. 3 or 4 tries and I managed to get enough in his eye to do some good.