If you want to buy or sell a couch, an old dresser, or some used tools, Craigslist is the place to go. Millions of people have found it to be a reliable and quick way to buy and sell almost anything–sex, too. Now, a report suggests these ads may be linked to HIV.

Those looking for a long-term affair or a quickie at lunch can log onto Craigslist in a number of U.S. states with city-specific entries and see a smorgasbord for any sexual taste.

Two researchers were curious about whether such easy availability to free sex would have a health impact on which states Craigslist comes to. Jason Chan of the Carlson School of Management at the University of Minnesota and Anindya Ghose of the Stern School of Business at New York University note the older ways of hooking up–want ads in newspapers, trolling the pick-up bars, sweet-talking the woman on the subway–were barriers to entry. With Craigslist, everything changed. With a few mouse-clicks, one could indulge in what Chan and Ghose describe as frequently “risky behavior.”

They shared their findings in “Internet’s Dirty Secret,” just published in the MIS Quarterly.

What they found is that whenever Craigslist shows up in a state, HIV rates skyrocket. They studied HIV rates in 33 states over the ten-year period from 1999-2008 and found a 15.9% increase in HIV cases. They wrote, “Our analysis suggest that the site entry produces an average of 6130 to 6455 cases of HIV infection in the US each year, mapping out to $62 million to $65.3 million in annual treatment costs.”

The authors contended, “In the East Central area, the annual number of new HIV cases was relatively stable from 2001 to 2004 [but that] an upward trend in HIV incidence is observed in the region as more Craigslist sites were launched post-2005 and is most acute when the number of personal ads increases past the 31,200 mark.” They observed the same phenomena in other regions of the country.

Craigslist came under ongoing pressure from activists and law enforcement for what many see as the promotion of illegal activity and even human trafficking. The online service dropped its “erotic services” section in 2009 when a masseuse was murdered after advertising on Craigslist. Even so, Craigslist still has a huge section devoted to those looking for that kind of action.

Much of the business formerly at Craigslist is now with competitor Backpage.com.