Miami (AFP) - Homeless men curl up on mattresses along a broken sidewalk littered with trash and syringes in Miami, a city struggling with the highest rate of new HIV infections of any big American metropolis.

Around the corner, Florida's first-ever needle exchange program opens Thursday, decades later than in many parts of the United States, and experts hope not too late to curb the burgeoning health crisis.

"We already have the perfect storm here in Miami," said Hansel Tookes, a 35-year-old doctor who began working several years ago to open a needle exchange in south Florida.

"We have a serious heroin epidemic. We were a cocaine town, but now we have this transition into opiate use," he added, referring to prescription painkillers that some inject to get high.

Miami's rate of new HIV infections -- 51.2 per 100,000 people -- far exceeds any other metropolitan area in the United States, according to data released this week by the US Centers for Disease Control and Prevention (CDC).

The nearly 1,400 new HIV infections last year in Miami -- a metro area including some 2.7 million people -- is about double the rate of all other big American cities, after adjusting for population size.

"Miami has overthrown Baton Rouge, Louisiana as the riskiest city for HIV," said Ethan Reynolds, a spokesman for GetTested.com.

And there is little help for those who inject drugs and want to stop.

"There are barriers everywhere," Tookes said.

"The system is built to make things more difficult for our patients."

- An old idea for a new problem -

The idea for needle exchange programs began in Amsterdam in 1983, with the goal of helping addicts avoid sharing dirty needles that spread blood-borne diseases, including hepatitis.

Users could bring their old needles to a clinic and trade them for new, sterilized ones.

Similar programs spread through the United States, beginning with San Francisco in the late 1980s.

But a limited number of cities have hosted needle exchanges. Critics have long held that the programs encouraged illegal drug use and did nothing to rehabilitate addicts.

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There was also a long-time ban on federal funding, which was effectively lifted in December 2015, when President Barack Obama signed a budget that allowed federal funds for syringe services programs, just not the needles themselves.

That funding change was "essential," said David Rosenthal, medical director of the Center for Young Adult, Adolescent and Pediatric HIV at Northwell Health in New York.

"Clearly, patients need to be ready to stop using drugs" for a needle exchange program to be effective, he said.

"Until individuals are ready to change their behaviors and stop using substances, we need to make sure we are giving them tools to keep themselves as healthy as possible."

- Rural surge -

A surge of HIV infections last year in Indiana -- nearly 200 in a rural community of 4,200 people -- shocked the nation.

The infections spread mainly among whites who were crushing painkillers and sharing needles to inject them among friends and family members.

Eventually, Indiana Governor Mike Pence -- who is now the vice president-elect -- reversed course by lifting a ban on programs that distribute clean needles.

This week, as World AIDS Day approached, CDC director Tom Frieden warned that "opiates threaten to undo the progress we have made preventing HIV over the past three decades," and called for wider use of needle exchanges.

Injecting drug users make up about 3,000 new HIV infections per year in the United States -- about nine percent of all new HIV cases in the nation.

But less than one percent of federal funds to fight HIV goes to needle exchange programs, Frieden said.

"The science shows that syringe services programs work. They save lives and money," he told reporters.

"And study after study has shown that they don't increase illegal drug use or crime."

- They won't be forgotten -

The University of Miami's new needle exchange is housed in two freshly painted trailers, stocked with syringes, tourniquets, cookers, condoms, blankets, food and free HIV tests.

Emelina Martinez has worked with people on the streets since the mid-1990s, doing HIV outreach, and said she is looking forward to helping at the new facility.

"These are my people," she said, smiling as she spoke of sharing hugs and stories with people who call her "Emy."

"I hope at least health-wise it helps a lot. With clean needles there is less infection, fewer wounds," she said.

"They will be healthier, just in general. And they won't be forgotten.

"So maybe it will inspire some of them to get better."