Two to three nights a week, he rubbed dirt in his hair and muddied up his jeans and shirt before walking the dark streets of Pittsburgh, searching for the very people he was trying to emulate.

Withers wanted to connect with those who had been excluded from his care.

"I was actually really shocked how ill people were on the street. It was like going to a third-world country," he said. "Young, old, people with mental illness, runaway kids, women (who) fled domestic violence, veterans. And they all have their own story."

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Homelessness costs the medical system a lot of money . Individuals often end up in emergency rooms, and stay there longer, because their illnesses go untreated and can lead to complications.

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For 23 years, Withers has been treating the homeless -- under bridges, in alleys and along riverbanks.

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"We realized that this was something that could be addressed. We could make 'house calls,' " he said. It's something that Withers' father, a rural doctor, often did.

Withers' one-man mission became a citywide program called Operation Safety Net . Since 1992, the group has reached more than 10,000 individuals and helped more than 1,200 of them transition into housing.

In addition to street rounds, the program has a mobile van, drop-in centers and a primary health clinic, all where the homeless can access medical care.

Today, Withers is also fostering a global "street medicine" movement. His nonprofit, the Street Medicine Institute , supports communities in starting programs of their own. Its network includes dozens of teams in the United States and around the world.

I recently talked with Withers about his work and his ultimate vision. Below is an edited version of our conversation:

CNN: How does it work, doing rounds in homeless camps?

Dr. Jim Withers: It can be a challenge to keep up with where the camps are. The homeless are often evicted. Their belongings are taken. And so they have to keep moving. And therefore we have to keep moving to keep up to date on their camps.

Withers and a student treat Lois Burkes, who his organization helped find housing.

Doing walking rounds, we try to keep our team down to four people. We don't want to invade (their space) with a lot of people. It's their home. So there's an outreach expert who knows the field and knows the people. There's myself or a medical person, and then we always have a student or two. And we bring a social worker with us.

We'll go into the camp, and we make sure they're OK with us coming in. Almost all the homeless are cordial, if not downright really friendly. And we just join them, and they can express what's going on. We always try then to check in with how they're doing medically. If they have insurance, if they have medicines that they should be on that they ran out of. Do they have a doctor? "Did you get that looked at?" What's important to the patient, the person, is our priority.

The care that we deliver out there is dependent on what's in our backpacks. We try to emphasize that this is not good enough. Getting (them) into primary care is our goal.

CNN: What ailments do you see out there?

Withers: People on the street, they're exposed to the elements obviously. So you see a lot of things that have to do with the weather and the difficulties of sleeping out there: frostbite, dehydration, hypothermia. But they also have everyday things, like high blood pressure, diabetes, coughs and colds, pneumonia, injuries.

I was actually really shocked how ill people were on the street. It was like going to a third-world country. Dr. Jim Withers

We've seen people with cancers. We've seen liver failure, people who needed dialysis that we actually took to dialysis from the street. It goes on and on and on.

We've seen almost everything that I've seen clinically and a few things that I hadn't. So it's a very rich environment for medical teaching, unfortunately.

CNN: You consider the streets to be a kind of classroom for students. What is their experience?

Withers: I see it in the eyes of the students that I take out. They see how many people are living in places that you'd never want to live. The weather, the rain, the cold.

I think they also get the chance to see success. When they can see a street person succeed and not just get locked in that hopeless category, their vision changes.

The street classroom really ignites, or reignites, what their passion is. They feel like, "Yes, this is what it's about." And they carry that forward. I think the lessons the homeless can teach us about finding the humanity and listening to people are transcendent in all of health care.

A plaque under a Pittsburgh highway commemorates the homeless men and women who have died.

Our students, I like to think of them as viruses. And we're spreading them out and they're beginning to go to their medical residencies, and they're saying, "Do you have street medicine here?" or "Why aren't you going under bridges?" And so the dialogue is really spreading.

CNN: What's your hope for the future?

Withers: In the way I'd like to see things, every person who is still on the streets will have medical care that comes directly to them and says, "You matter." And I'd love to see every medical school have a classroom of the streets.

I've dedicated myself as much as I can to finding anyone, anywhere, who's interested in doing medical care to their own street people and helping them. Besides just the good that it does and the money that it saves, having street medicine in every community transforms us. We begin to see that we're all in this together.

It's something that we should take pride in when we can actually treat people the way we would want to be treated.