Last week I wondered why humans are so uniquely fragile. Since then I've read Elizabeth Kolbert's book The Sixth Extinction: An Unnatural History, and she has something to say that might be related. With regard to humans causing the extinction of prehistoric megafauna, she notes that species gain from gigantism by being too large to have a predator. Such animals have longer gestation periods, which makes them more susceptible to extinction. (I'd reference page numbers but I already returned it to the library, so you'll just have to trust me: it was in the last couple chapters.) So humans are threatened by childbirth because we have giant babies so our adults are too big for most predators to eat.
Meh, maybe. But why does every wound get infected and kill us? It's not just childbirth fragility that seems odd. Another explanation from Kolbert's book might be that these viruses and bacteria have emerged too recently for our immune systems to have learned how to fight them, but wouldn't that be true for all animals' immune systems? Why are we alone in dying from wounds at so high a rate?