Prey by Michael Crichton

In the Nevada desert, an experiment has gone horribly wrong. A cloud of nanoparticles -- micro-robots -- has escaped from the laboratory. This cloud is self-sustaining and self-reproducing. It is intelligent and learns from experience. For all practical purposes, it is alive.
It has been programmed as a predator. It is evolving swiftly, becoming more deadly with each passing hour.

Every attempt to destroy it has failed.

And we are the prey.