An analysis of over 80,000 such web pages shows that nine out of ten visits result in personal health information being leaked to third parties, including online advertisers and data brokers.
This puts users at risk because their health interests may be publicly identified along with their names, researchers said.
This could happen because criminals get hold of the information, it is accidentally leaked, or data brokers collect and sell the information, they said.
Given that 62 per cent of bankruptcies are the result of medical expenses, it is possible anyone visiting medical websites may be grouped into the "waste" category and denied favourable offers.
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Given that data brokers are free to sell any information they collect regarding visits to health websites, those visiting such sites are potentially at risk of being discriminated against by potential employers, retailers, or anybody else with the money to buy the data.
Libert said a software tool can investigate Hypertext Transfer Protocol (HTTP) requests initiated to third party advertisers and data brokers.
He found that 91 per cent of health-related web pages initiate HTTP requests to third-parties. Seventy per cent of these requests include information about specific symptoms, treatment, or diseases (AIDS, Cancer, etc).
The vast majority of these requests go to a handful of online advertisers: Google collects user information from 78 per cent of pages, comScore 38 per cent, and Facebook 31 per cent. Two data brokers, Experian and Acxiom, were also found on thousands of pages.
"For those who use Google's social media offering, Google+, a real name is forcefully encouraged. By combining the many types of information held by Google services, it would be fairly trivial for the company to match real identities to "anonymous" web browsing data," Libert added.