Plague Year (Plague, #1) by Jeff Carlson

The nanotechnology was designed to fight cancer. Instead, it evolved into the Machine Plague, killing nearly five billion people and changing life on Earth forever.

The nanotech has one weakness: it self-destructs at altitudes above ten thousand feet. Those few who’ve managed to escape the plague struggle to stay alive on the highest mountains, but time is running out-there is famine and war, and the environment is crashing worldwide. Humanity’s last hope lies with a top nanotech researcher aboard the International Space Station-and with a small group of survivors in California who risk a daring journey below the death line…