The behavior of nonlinear dynamical systems has been the unifying theme of my own nonlinear academic trajectory. Beginning as an undergraduate chemical engineer, I ended up with a PhD in theoretical physics, and roughly 10 years later transmogrified into a professor of biology at Princeton University. I believe the ways in which system risks can arise, and propagate, in different settings is best seen from many different perspectives. And it is increasingly clear that such a view of complex adaptive systems is critical to our future well-being, as we are indeed engulfed in complex, and often coupled, systems, from our environment to our social networks and our financial systems.

HIV seems to have learned a thing or two from Proteus. It constantly changes form, eluding the immune system much the way that mythological sea creature evaded Menelaus. Although a person typically gets infected with a single strain of the virus, after a decade of infection two HIV viruses in the body may differ by as much as 10 percent — a greater difference than that between the key regions of mouse and human DNA. So before the immune system has managed to grasp the first form of HIV, the virus has changed form enough to become unrecognizable.

Eight centuries ago, the Four Corners region of the US Southwest was bustling. Regular rainfall coaxed crops from healthy soil, and the abundance of cottontails, jackrabbits, and mule deer made for choice meals. Tens of thousands of Ancestral Puebloans lived in adobe houses and cliff dwellings spread across an area the size of Napa County. Then, in the final decades of the 1200s, most everyone left. Drought, crop failure, and conflict all contributed to the society’s collapse. But we don’t exactly how the migration