we had one sub adult female at our store that came from a breeder who kept fairly high humidity, our store humidity is rather low and takes some creative enclosure building to keep up...any how this female developed an RI in less than two weeks.
I took her and set her up in a tub and go the humidty back to the breeders settings and she delt much better with it. The RI did still take a series of shots to clear up but being back in the higher humidty was definatley a key to her recovery. She was back to eating and living well after about 2 weeks.
Fore sure humidity has something to do with Balls developing a IR. Our first two years in our new building we had some females get IR and we just couldn't figure it out until we took a look at our relative humidity. We were down in the low 20% range. We since have fixed that problem and we have a small percentage of IR that we had the first two seasons. There are many other things to consider as well, stress, temps, husbandry, etc... But humidity is one for sure. Hope this helps, Brian(BHB)