After Craigslist entered the market, researchers from the University of Minnesota saw a 15.9% increase in HIV cases. Nationwide, that’s 6,000 cases annually and between $62 million and $65.3 million in costs.

Jason Chan, Assistant Professor of Information and Decision Sciences at the Carlson School of Management, and Professor Anindya Ghose of NYU’s Stern School of Business analyzed data from 1999 to 2008 and said that Craigslist’s random entry into the market with regard to other markets that were affected by HIV made for a perfect natural experiment.

“I actually think that the creators of Craigslist had no intent of harming society. They came in with good intentions,” Chan said. “At the same time, they did not anticipate that users could use the features in an unexpected way with unintended consequences.”

Chan and Ghose believe that the rise in HIV cases is due to personal ads placed on the site and not official escort service ads. This is backed up by data suggesting that internet-facilitating sex workers tend not to take risks with clients.

“Our study results suggest that there is a new social route of HIV transmission that is taking place in this digital era,” Chan said. “Health care practitioners and policymakers have to look more closely at online platforms to assess how its usage may facilitate the spread of HIV and STDs across the country.”