In “The Sussex Vampire,” Sherlock Holmes receives a plea for counsel from Bob Ferguson, an Englishman whose Peruvian wife has become dangerously ill after being discovered in the bizarre, apparently murderous act of sucking blood from their infant son’s throat. “What can I do?” Ferguson asks. “How am I to go to the police with such a story?” As seems reasonable, he fears for her sanity. “Is it madness, Mr. Holmes? Is it something in the blood?” The detective immediately