Plague Series 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... moreThe 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... less

Researcher Ruth Goldman has developed a vaccine with the potential to inoculate the world’s survivors against the nanotech plague that devastated humanity. But the fractured U.S. government will stop at nothing to keep it for themselves.

First Earth was devastated by the machine plague, a runaway nanotechnology that devoured all warm-blooded organisms below altitudes of ten thousand feet. Then the remnants of humankind turned on one another, provoking a brief, furious world war and the invasion of North America. Now Russia and Chinese armies hold California against the battered forces of the U.S.-Canadian Alliance... moreFirst Earth was devastated by the machine plague, a runaway nanotechnology that devoured all warm-blooded organisms below altitudes of ten thousand feet. Then the remnants of humankind turned on one another, provoking a brief, furious world war and the invasion of North America. Now Russia and Chinese armies hold California against the battered forces of the U.S.-Canadian Alliance.Nanotech researcher Ruth Goldman and Cam Najarro - a former Army Ranger who helped her force an end to the war - have finally found some peace in a small, hidden village in the Rockies. But the arms race for weaponized nanotech has continued, and America is struck by a new contagion.Together with a small band of friends and rivals, Ruth and Cam must discover the source of the new plague - never suspecting that its creator is an old enemy they believe dead... less