It's pretty clear to me that the disease hasn't progressed far enough to make him a full-on killer when he sees Brett. It's another day at least before he starts attacking people.

Ya or TAD either

Tad might not have been attacked due to rabies making Cujo unpredictable. Concerning his owner, Cujo was in the beginning stages of rabies. If you have read the book, the narrator has this part as if Cujo was thinking and being in a confused state in which he wanted to kill his owner due to the disease, but also remembering how much loyalty and love he had for his owner. He then runs away.