Craigslist is the place where numerous communities conduct the everyday business of life: Locating affordable apartments, selling household items, buying cars and, yes, finding people with whom to have sex.

Researchers at University of Minnesota and New York University have linked the latter type of transaction, facilitated thousands of times each day via Craigslist’s personal ads, to a 15.9 percent increase in HIV infections after the site moves into a new U.S. market.

Their findings were published in the December edition of MIS Quarterly.

The study — titled, in part, “Internet’s Dirty Secret” — found that Craigslist transactions are linked to more than 6,000 cases of HIV infection nationwide each year, resulting in as much as $65.3 million in annual treatment costs.

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

Chan, now an assistant professor of information and decision sciences at University of Minnesota’s Carlson School of Management, told the Minneapolis Star Tribune that the idea for the study originated while he was a doctoral candidate at New York University and read about a doctor “who traced sexually transmitted diseases to online chat rooms.”

“I went to Craigslist in New York and went through the ads,” said Chan, who wrote the journal article with Anindya Ghose, a professor at NYU’s Stern School of Business. “To my shock, I found out almost all the ads were hookup ads. They were very explicit.”

Chan and Ghose analyzed data in 33 states between 1999 and 2008, according to the study. Because Craigslist enters a community with existing HIV trends, researchers were presented with a unique opportunity to measure how the site seems to influence a given market after its arrival.

The study notes that a one-year delay exists between the arrival of Craigslist and an increase in HIV cases in a particular community. Chan attributes the delay to to the time it takes for the site’s local popularity to grow and for infected individuals to get a diagnosis, according to the Star Tribune.

“In particular,” the researchers wrote, “we argue that the introduction of Craigslist provides users with the ability to locate and socialize with specific groups of individuals who are inclined to engage in casual sex but are otherwise hard to locate in one’s day-to-day life and community.”

Craigslist did not respond to The Washington Post’s e-mails and phone calls seeking comment on the study’s findings.

After accounting for factors such as increased testing in a community, researchers discovered that the increases in HIV infections were influenced by the site’s personal ads — not professional escort services, which were eliminated from the site in 2010 amid pressure from law enforcement, according to Bloomberg.

Though many of those services have tried to camouflage themselves by migrating to personal ads, Chan told the Star Tribune that he does not attribute the increase in HIV infections to sex workers, who he believes are more likely to take precautions than those seeking casual sex.

Surveys reveal that individuals who seek out casual sex partners online have multiple sex partners and do not use condoms, according to the study. “Significantly more” of those individuals also engage in unprotected anal intercourse, have sexual exposure to HIV-positive individuals and report less worry about HIV infection, thanks to improved treatment options, according to the study.

How do personal ads work? The study explains:

A typical personal ad post on Craigslist consists of an open-ended text requesting for meet up, a telephone number, a randomly generated Craigslist email address, and photographs of the poster. Personal ads are classified into subcategories that denote the relationship type that users are seeking for (i.e., men seeking men, men seeking women, women seeking men, women seeking women).

The study was unable to distinguish between which categories of personal ads were most closely linked to increases in HIV, but Chan told the Star Tribune he thinks he knows.

“My suspicion is most of this [effect] is coming from the M for M section,” he said, referring to personal ads under the category “men seeking men.” “We do know that anal sex is a large reason why HIV is spread. It’s also prevalent in the gay community. It makes sense that this group is the most likely to be affected.”

In 2010, the most recent year data are available, the Centers for Disease Control and Prevention estimated that 1,144,500 people were living with HIV in the United States, including 180,900 (15.8 percent) people whose infections had not been diagnosed. The agency estimates that 50,000 people become infected with HIV each year.

The researchers mapped increases in HIV cases by region. In some regions, they were able to show that an upward trend of HIV incidence was most acute when the number of personal ads reached a certain mark, such as 31,200 in the “East Central area.” In other regions, the HIV cases were falling until “activity levels on Craigslist sites grew after 2003,” and “HIV incidence began to rise accordingly.”

Chan said that the study suggests that HIV transmission is evolving with the digital era.

“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,” he said.