“You make me sick” is a colloquialism, but it reflects a reality. Our health depends not just on our own biology, choices, and actions but also on the biology, choices, and actions of those around us.

This claim may strike some as anathema. Particularly in the United States, we are accustomed to seeing our destinies as largely in our own hands. We “pull ourselves up by our bootstraps.” And we have a “do it yourself” culture that clearly extends to our own health. The radical individualist perspective is that by making changes in everything—from what we eat to how we exercise, how we brush our teeth, when we sleep, and whether we seek regular check-ups—we can improve our survival chances, mental stability, and reproductive prospects.

But the picture is much more complicated. Our unavoidable embeddedness in social networks means that events occurring to other people, whether we know them or not, can ripple through the network and affect …