In April 2014, Tim Libert, a researcher at the University of Pennsylvania, custom-built software called webXray to analyze the top 50 search results for nearly 2,000 common diseases (over 80,000 pages total). He found the results startling: a full 91 percent of the pages made what are known as third-party requests to outside companies. That means when you search for “cold sores,” for instance, and click the highly ranked “Cold Sores Topic Overview WebMD” link, the website is passing your request for information about the disease along to one or more (and often many, many more) other corporations.

Looking Up Symptoms Online? These Companies Are Tracking You | Motherboard.