A.The correct answer to this question is beyond the scope of one definitive answer. Most Cushing’s cases are secondary to a benign tumor in the pituitary gland; currently, surveys show that approximately 50% of all disease-related deaths in dogs are due to cancer. So this is almost analogous to trying to explain the cause of cancer in one paragraph! Basically, immune suppression and aberration are behind it all, so we must look at the many factors that cause this. And yes, vaccinations are one of those key factors.