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A tracking tool installed on many hospitals’ websites has been collecting patients’ sensitive health information—including details about their medical conditions, prescriptions, and doctor’s appointments—and sending it to Facebook.

The Markup tested the websites of Newsweek’s top 100 hospitals in America. On 33 of them we found the tracker, called the Meta Pixel, sending Facebook a packet of data whenever a person clicked a button to schedule a doctor’s appointment. The data is connected to an IP address—an identifier that’s like a computer’s mailing address and can generally be linked to a specific individual or household—creating an intimate receipt of the appointment request for Facebook.