From the newsweek article at
Christopher Voigt, of UC San Francisco, and Christina Smolke, of Caltech, are in the early stages of designing microbes that would circulate through the human bloodstream, seeking out cancerous tumors anywhere in the body. The microbes might be equipped with a biodevice that detects the low oxygen levels characteristic of a tumor, another that invades the cancer cells, a third that generates a toxin to kill the cells and a fourth that hangs around afterward in case the cancer comes back. All this would happen without the patient's even knowing. Eventually, circulating cellular sentries could monitor and adjust blood levels of critical substances including glucose and cholesterol.
Now that's Cool!