No insulin means the body’s blood sugar refinery is effectively shut down. As of today, Type 1 diabetes can only be treated with daily doses of insulin, typically through self-administered injections, although it is also possible to have an insulin pump surgically implanted. On top of daily insulin doses, Type 1 requires vigilant attention to blood sugar levels and a superdisciplined approach to diet and exercise. In Type 2 diabetes, the pancreas still produces insulin—sometimes even at high levels—but the level of insulin production can eventually be too low or other tissues in the body are resistant to it, impairing the absorption and conversion of blood sugar.

I hope it gives you a window into the beautiful, varied, and interconnected nature of life on this wonderful world we inhabit. Instead of just asking what’s wrong and what can be done about it, I want people to look behind the evolutionary curtain, to ask why this condition or that particular infection occurs in the first place. I think the answers will surprise you, enlighten you, and—in the long run—give all of us a chance to live longer, healthier lives. We’re going to start by looking at some hereditary disorders.

Given that iron is not only crucial to our survival but can be a potentially deadly liability, it shouldn’t be surprising that we have iron-related defense mechanisms as well. We’re most vulnerable to infection where infection has a gateway to our bodies. In an adult without wounds or broken skin, that means our mouths, eyes, noses, ears, and genitals. And because infectious agents need iron to survive, all those openings have been declared iron no-fly-zones by our bodies. On top of that, those openings are patrolled by chelators—proteins that lock up iron molecules and prevent them from being used.