The introduction of Craigslist led to an increase in HIV-infection cases of 13.5 per cent in Florida over a four-year period, according to the study conducted at the University of Maryland.
Online hookup sites have made it easier for people to have casual sex - and also easier to transmit sexually transmitted diseases, researchers said.
The new study measured the magnitude of the effect of one platform on HIV-infection rates in one state, and offered a detailed look at the varying effects on subpopulations by race, gender and socio-economic status.
The study "underscores the need for broader communication and dissemination of the risks posed by the type of online matching platforms studied here," said Ritu Agarwal, professor at the university's Robert H Smith School of Business.
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It also found that the new HIV cases came disproportionately from one racial-ethnic group, African Americans, who accounted for some 63 per cent of the new cases.
"That is a bit of paradox because research suggests that the African American community is one which uses the Internet the least, even though the gap is narrowing," said Agarwal.
For patient data, the researchers drew on a census that included data on some 12 million patients; omitting institutions that had no HIV cases or which were open for only part of the period studied, there were 223 hospitals in the sample.
There was also an increase in new HIV cases among Latinos and Caucasians - although only intermittently statistically significant and not statistically different from each other.