According to Wired, Google X is working on nanotechnology that can swim around in your blood and tell you what it finds:

“Because the core of these particles is magnetic, you can call them somewhere,” Conrad said, indicating that you could use a wearable device to gather them in the superficial veins on the inside of your wrist. “These little particles go out and mingle with the people, we call them back to one place, and we ask them: ‘Hey, what did you see? Did you find cancer? Did you see something that looks like a fragile plaque for a heart attack? Did you see too much sodium?”