Postulating that “the ease of seeking sex partners through classified ad sites may promote risky behaviors that increase transmission of STDs,” the researchers reviewed HIV surveillance data from 33 U.S. states between 1999 and 2008 and compared that with the arrival of Craigslist personals in various markets.

After factoring out other explanations for the increases, the investigators concluded that Craigslist likely played a role in a nearly 16 percent rise in HIV cases in those locales.

Jason Chan, assistant professor of information and decision sciences at the Carlson School of Management at the University of Minneapolis and the study’s lead author wrote, “Our analysis suggests that the site entry produces an average of 6,130 to 6,455 cases of HIV infection in the U.S. each year, mapping out to $62 million to $65.3 million dollars in annual treatment costs.”

The study also concluded that it was casual hook-ups, not sex worker transactions that were responsible for the rise. This seems in line with previous findings that sex workers who find their clients online are less likely to participate in risky sexual practices with those patrons.

In a press release, Chan called on health care practitioners and policymakers to “look more closely at online platforms to assess how its usage may facilitate the spread of HIV and STDs across the country.”

The paper’s title, “Internet’s Dirty Secret: Assessing the Impact of Online Intermediaries on HIV Transmission,” has struck some in the field as unusually “judgmental.”