There's a fascinating phenomenon in medicine of a "carrier" of a disease. The most famous example is Typhoid Mary. Mary was a cook in the early 20th century who spread typhoid to over fifty people, although she never contracted the infection herself. There are carriers of a variety of illnesses. For instance, a person can test positive for TB without having suffered from the disease. Typhoid Mary is, I think, an apt metaphor for not just carriers of physical disease, but of psychological mayhem. I'm referring to people who wreak havoc without so much as lifting a finger. The most...