Medical data is legally shared with more third parties in health care than many Americans realize--and it can cause potentially deadly mistakes.

In a year when former National Security Agency contractor Edward Snowdens revelations about the collection of U.S. phone data have sparked privacy fears, data miners have been quietly using their tools to peek into Americas medicine cabinets.

Until recently, medical files belonging to nearly 300,000 Californians sat unsecured on the Internet for the entire world to see.