Katie Wu is a bona fide cat person. She has two of them: twin boys named Calvin and Hobbes. Every night, they curl up in bed with her, bonking their little noses together, rubbing their fur and whiskers everywhere, and leaving behind inevitable cat residue. It's certifiably cute ... and a little bit gross.It's also the worst nightmare for the cat-allergic. Which, just shy of a decade ago, Katie was. In a stroke of luck, Katie's debilitating cat allergy disappeared. The reasons for her immune overhaul remain a mystery.Allergies can wax and wane over time, but it seems to be less common to have the night-and-day shift that Katie experienced. In this episode, Katie walks host Aaron Scott through the dynamic world of allergies and what it reveals about our immune systems. And of course, Katie's cats make cameo appearances. (encore)