Why do other people get sick so much?

No, really.  I had a cold in November.  Before that, the last time I was sick was…three years ago, in May?  I’ll grant, that was the flu, but still.

I don’t think of myself as an exceptionally healthy person, but apparently my immune system is really, really robust.  Because I have a number of friends who’re always saying they’re mildly ill, and everyone else seems to think this is perfectly normal.