Want to cut down on HIV infection rates? Decriminalize prostitution.

That's the finding of a new study published in The Lancet. The study found that decriminalizing sex workers could have a huge effect on the HIV epidemic, averting between one-third and nearly half of infections over the next decade.

FSW=female sex workers; ICU=inconsistent condom use. Image Credit: The Lancet via Vox

How it works: Female sex workers are especially at risk when it comes to the virus. In Vancouver, 12% of those who work outdoors and 20% of those who work informally indoors have HIV, according to Vox.

In places where their trade is illegal (which is most places), sex workers must work in secret, avoiding police and having less control over their clients. Violence from clients, according to the study, is associated with less condom use. Less condom use, of course, is associated with HIV.

HIV aside, stopping violence from clients is its own reward. There's also police harassment, which affects 70% of outdoor and informal indoor sex workers. More police harassment (anything from "police raids to coercion and abuse," as study author Kate Shannon told Vox) drives prostitutes to more out-of-the-way locations and gives them less control over their clientele, upping their risk factors.

Positive evidence: To see the beneficial effects in action, one need only turn to Rhode Island. The state accidentally decriminalized prostitution — it's a long story — leading to six years of unfettered business as long as the work was done indoors.

"Decriminalization caused both forcible rape offenses and gonorrhea incidence to decline for the overall population," a study revealed earlier this month. "Our synthetic control model finds 824 fewer reported rape offenses (31% decrease) and 1,035 fewer cases of female gonorrhea (39% decrease) from 2004 to 2009."

Image Credit: Vox

Criminalized or not, prostitution is a $14 billion a year business in the United States alone. That's a lot of sex workers whose jobs present a lot of risks, HIV included. Canada is considering decriminalization this year after its Supreme Court struck down outdated prostitution laws.

In the studies above, at least, the numbers are clear — decriminalized sex work means less violence, less police harassment and fewer infections.