First Pet Stuff "Pet is Busy" Sign Question

I just got My First Pet Stuff and am messing around with the different interactions available. I want to see how the Rabid Rodent Fever pans out firsthand, so I am having my one sim be mean to their pet hamster. The two others in the household still have a good relationship with him though. Well, I got the mean sim's friendship with the hamster to enemies and she got bit shortly after that. No fever, so I was going to have her play with the hamster again. Except for no reason he put up a sign saying he was busy and to come back later. I read the small pets will do this if your sims go on vacation, but no one left. Is he is just fed up with being picked on? Also, does the cage have to be dirty or does it only increase the chance of getting the fever. Thanks in advance. :smile: