There are many examples of what Casanova calls “the infection enigma.” Herpes simplex viruses, for instance, can cause annoying cold sores and genital lesions in many people, but in rare cases they invade the brain and incite potentially lethal encephalitis. Candida albicans is a ubiquitous fungus that typically causes serious harm only to people with a weakened immune system, and yet some otherwise healthy individuals suffer repeated bouts of infection. Even with a dire disease such as tuberculosis, Casanova notes, in places where the TB bacterium is endemic, “everyone inhales it, but not everyone gets sick, and fewer than one in 100 will die of TB.” This can’t just be the fickle finger of fate, he reasoned.