Andrey SuslovGetty Images Almost all of us can report those eerie “coincidences”—targeted ads on social media for, say, open-toed wedge sandals when you’d swear you were just daydreaming about a vacation someplace warm. Something in your search history or shopping patterns told the Facebook gods that you’d click on an image of shoes. It’s weird, you may think, but no big deal. Companies are increasingly discovering, however, that there’s gold to be mined in a topic much closer to home and with more potential to harm you than footwear preferences: your health. Amassing personal medical information has become a huge business in recent years, with medical centers and data brokers selling or sharing everything from your diagnoses to lifestyle habits and activities that could raise your risk for future diseases.