Appendicitis is a weird illness. It’s more common in the wealthy world than the developing world, even though it doesn’t seem to be lifestyle based. Poor people in wealthy countries have the same low appendicitis rate. It’s more common now, all over the world, than it was fifty years ago but it isn’t contagious from person to person.

We used to blame the appendix for this. We called it vestigial, and removed it at the drop of a hat. Charles Darwin himself told us it no longer had a purpose.  We don’t need it to digest, and you don’t find it in other mammals.

We’ve got new research, though, and it’s not the appendix causing the trouble – it’s our modern lives. The appendix only started getting useless about a hundred years ago. It’s a reservoir for the healthy bacteria we need to recover from revolting intestinal ailments like diarrhea. When we don’t get sick that often, however, the immune system doesn’t have enough to do. Appendicitis is one result; allergies are another.

That’s also why appendicitis rates are lower in the developing world. Without consistent access to clean water, our immune systems – and our appendixes - have work to do.  They help us fight off cholera and dysentery instead of getting all wound up and attacking our own bodies.