Because of their rapid development and rare occurrence, certain diseases are difficult to diagnose. Often the anamnesis of such cases is lacking because the owner does not observe anything before he finds that one or more animals in the herd has succumbed. Such cases of rare diseases in a community are puzzling to the owner as well as to the veterinarian. Furthermore, it is probable that a rare enzootic may easily escape attention because of failure of a diagnosis. Aujeszky's disease falls into this category since it is sporadic in occurrence, concealed in its development, and of short duration before death ensues in the affected animals. In species such as the pig, the course of the disease may be insidious and the recovery uneventful.