The RV arrived at a corner near D.C.'s Marvin Gaye Park, also known to locals as "Needle Park." A steady procession of addicts came to the door, mounted a few steps and sat down. One by one, they dropped used needles into a container and received new needles in return, along with alcohol wipes and the small, bottle-cap-like "cookers" in which heroin is heated.

Reggie Jackson, Teefari Mallory and Hazel Smith – staff members at PreventionWorks, Washington's largest needle-exchange program – are at the park twice a week, offering clean needles to prevent disease transmission, condoms, drug treatment referrals, HIV/AIDS testing and a few kind words. "You still play the guitar?" "You'll have a swollen hand if you keep going there." "Love you, baby."

It is the eyes and arms of addicts that draw your attention. Eyes that are glassy, or unnaturally bright, or tired beyond exhaustion. Arms that are ulcerated sticks or purpled parchment; with repeated use, needles become blunt and tear the skin. Some addicts adopt a defensive politeness – "Yes, sir" – and quickly leave. Others want to talk – "I love plants, and I love kids" – trying to provide hints of their humanity. They are America's least wanted.

They are also at the center of a controversy. Needle-exchange programs have always been politically controversial, with opponents arguing that they send a mixed moral message about drug use. The House of Representatives recently passed an amendment banning exchanges in the District within 1,000 feet of places where children gather – which, if approved by the Senate, would effectively put programs such as PreventionWorks out of business. Staffers joke that they could work only in graveyards and the middle of the Potomac.

This restriction might make sense if needle-exchange programs increased the number of addicts. But they don't. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases at the National Institutes of Health, has comprehensively reviewed the scientific studies on needle exchange. "It does not," he says, "result in an increase in drug abuse, and it does decrease the incidence of HIV. … The idea that kids are going to walk out of school and start using drugs because clean needles are available is ridiculous."

My experience in Washington was consistent with Fauci's view. Addicts who came for needles were generally in their 40s and 50s. The availability of clean needles no more caused their addiction than the provision of clean shot glasses would cause alcoholism.

The main purpose of needle exchange, according to Jackson, the supervisor of the mobile unit, is to keep people alive until they can get clean – a process that can take years, if it happens at all. Needle-sharing is the third-leading cause of HIV infection in our nation's capital. It is also a major contributor to the spread of hepatitis C, the main cause of liver transplants in the United States. Jackson is well acquainted with these facts because, while an addict, he contracted both diseases. "If they had a truck like this in the '60s, '70s and '80s," he told me, "maybe I wouldn't have gotten infected."

The staff members of PreventionWorks build long-term relationships with people no one else knows by name. Because of this, they have a good feel for when addicts are ready for treatment. While I was in the RV, Jackson signed up two addicts for detox. Mallory used her own car to drive one addict, with whom she had been working for eight years, to treatment. "He's ready, ready to go," she said, fighting tears.

Critics claim that needle-exchange programs create a moral hazard by legitimizing drug abuse. But it does not legitimize drug abuse to help people with the clinical disease of addiction avoid other deadly diseases until they are ready for help. Sacrificing the lives of addicts to send an "unmixed" moral message actually sends a troubling moral message: that the unwanted have no worth.

As each addict leaves the RV, Smith – who was an addict on the street herself four years ago – tells them: "I love you." When I asked her why, she said: "If someone years ago had told me they loved me, it might not have been so long."

Street addicts are connected to the rest of us by only a few invisible strands – people such as Smith, Jackson and Mallory – and those strands should not be severed.

Michael Gerson writes for the Washington Post Writers Group. Reach him at mgerson@ globalengage.org. Cal Thomas will return next week.