I think what is indicated when, in spite of being injected with a pathogen isolated from a sick animal, another animal remains healthy; is that the animal could be immune to that particular pathogen or may be the pathogen was not pathogenic. A pathogen is an agent that causes a disease;;according to Koch postulates when isolated pathogen is injected into a healthy animal, animals must develop disease, which if the disease is not developed, then the pathogen is not pathogenic or may be the animal is immune.