Every now and then one pops up at work, down the pub, in the park, outside the school gate, or in your own family’s mythology. The person who claims never to get sick. Colds brush past them without leaving so much as a sniffle. They laugh in the flushed face of flu, spray hand sanitiser in the rheumy eyes of infection, and never take a day off work. They appear to be superhuman, with the kind of kickass immune systems the rest of us mere ailing mortals can only dream about as we dissolve another 1,000mg vitamin C tablet and hope for the best. What are their secrets? Can we become more like them? Do they even exist?