From general pain and infection, which in Miss Meow’s case was from badly infected teeth. When I got her she had problems with her mouth & skin infections, and I thought that after a couple of rounds of antibiotics and having four teeth removed that had been resolved, but it turns out the infection in the teeth was still there, and progressed over the course of a year to the point where she had to have all her teeth removed (it turns out she is FIV positive, which is why she was so succeptible to the infection). Not having a baseline for “normal” behaviour for her I thought her lack of interest in toys was just her personality, but she is much more active and playful now.