As I write this article, I have a cold. At some point in the past couple of weeks, I managed to inhale a droplet of water suspended in the air that contained hundreds or thousands of copies of a virus (probably a rhinovirus, the kind that causes most common colds). That virus infected my throat and my sinuses, resulting in the sore throat and stuffed-up nose I have right now. As inconveniences go, this feels rather minor. I think of my friends with small children, who are constantly careening be...