The Hate Disease ♦ By Murray Leinster

Dr. Calhoun and his pet tormal Murgatroyd work for the Interstellar Medical Service making routine public health inspections on far-flung colonial planets. When they reach Tallien Three they are greeted with a rocket attack by the Paras, a mutated form of human rapidly replacing the “normals”. The normals think it’s a pandemic of demonic possession but Calhoun has his doubts. If he can keep from turning into a Para, or being assassinated by them he just might figure this thing out.