Craigslist's entry into a market results in a 15.9 percent increase in reported HIV cases, according to research from the University of Minnesota published in the December issue of MIS Quarterly. When mapped at the national level, more than 6,000 HIV cases annually and treatment costs estimated between $62 million and $65.3 million can be linked to the popular website.

"I actually think that the creators of Craigslist had no intent of harming society. They came in with good intentions," says Jason Chan, Assistant Professor of Information and Decision Sciences at the Carlson School of Management. "At the same time, they did not anticipate that users could use the features in an unexpected way with unintended consequences."

Chan and Professor Anindya Ghose of NYU's Stern School of Business based their claims after analyzing data in 33 states from 1999 to 2008. Because Craigslist randomly enters individual markets with respect to HIV trends, it provided a unique natural experiment setup from which the researchers could uncover the connection.

After conducting a series of tests to eliminate other possible causes that might be driving the HIV trends such as increased testing in a community, the researchers discovered that the upward shift was influenced by ads in Craigslist's personals sections, not the site's escort service ads. This finding was in line with existing research that shows internet-facilitated sex workers are less likely to participate in risky sexual practices with clients.

"Our study results suggest that there is a new social route of HIV transmission that is taking place in this digital era," says Chan. "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."

According to Chan, the paper provides practitioners with insights on how they can effectively target their efforts to reduce disease transmission facilitated through classified ad sites. The Center for Disease Control has taken note of the findings and posted an early version of the paper its website.