Disease is a word that carries such a visceral meaning: we've all had (or still have) one at some point, and have experienced the pains and challenges that come with it. They always reminds us of mortality and how fragile we all are and keeps us on our toes.
Some diseases, however, are actually a design made to increase your longevity and this books points out several of them and goes deep in explaining the science behind them and what we know about human physiology. It is not restricted to diseases, though, as it also explains some ethnic differences that also improve a person's longevity.
It also talks about how your body adapts to certain conditions to achieve longevity. I was specially amused by his explanation on how the Wood Frog manages to freeze and halt his whole body to a complete stop during winter and thaw and come back to full functionality during spring, and how some of that related to our own body as well as our mechanisms to regulate temperature and how it works to protect internal organs by making some sacrifices.
Also interested in the frog part? Here is a neat video about it, although the book does a much better job at really explaining what happens during the freeze/thaw process.
This is certainly one of my top books for the curious person, and I highly recommend it.