I think of myself as a healthy person, but airborne contagion doesn’t care what I think, and it doesn’t care if I go to fitness class or run marathons. It’s an equal opportunity disrupter. A neurologist once told me that some people are more vulnerable to diseases of the central nervous system than others. He thought I was one of them because in my time on the planet I have uploaded viral meningitis, polymyalgia rheumatica, transient global amnesia, two episodes of myasthenia gravis and what a pediatrician diagnosed as a mild case of polio. I’m lucky that they’re all in the rear-view mirror now.