The key, as IBM is doing, if working with a large HMO or health care network who hopefully have switched to a sensible EMR and have built up a good amount of historical data on patients.
I'd add one last key to getting this right isn't only the breadth of the data, but the depth. Knowing some superficial aspects of a person (age, weight, habits) is too naive. You need family history, you need psychosocial aspects (nightmares, trouble at work, marital problems etc.). If you can get that, then you're cooking.