JUDY WOODRUFF: But, first: a bloody year in Chicago. And residents, police and community leaders are asking why the violence is getting worse, instead of better?

It is already the deadliest year in more than two decades, 500 homicides so far, 90 in August alone. The killings were mostly clustered on the city's South and West Sides.

John Yang went to Chicago to try to find out why, despite calls for new action, the violence there is so hard to rein in.

JOHN YANG: On this busy street corner in Englewood, one of the hardest-hit neighborhoods on the city's troubled South Side, it looks like a party. Kids are playing. The grill is fired up. In the past, though, 75th and Stewart felt like a war zone.

TAMAR MANASSEH, Founder, Mothers Against Senseless Killings: This corner is a corner where a man was killed — well, several men, a woman was killed, and a child was killed. A 9-year-old girl was killed right across the street over there washing her dog in broad daylight.

JOHN YANG: But for two summers, a group called Mothers Against Senseless Killings, or MASK, led by Tamar Manasseh, has been out on this corner, and there hasn't been a single shooting.

Volunteer Laura Lambert (ph) comes from nearby Hyde Park. And 91-year-old Edwina Knight (ph) crosses the street every day from the house she's lived in for 57 years.

TAMAR MANASSEH: Just show up. That's all you have to do. Show up, grab a lawn chair and a pair of sunglasses, and you can do this. You can change the world with that.

JOHN YANG: But the moms of MASK are only on one corner in a city of 2.7 million people. Killings have spiked this summer. Chicago has already recorded more homicides this year than it did in all of last year.

DR. GARY SLUTKIN, CEO, Cure Violence: This is also very typical of all epidemics. You can see the high density of shootings.

JOHN YANG: University of Illinois at Chicago physician Gary Slutkin says epidemic is exactly the right word. He argues that violence is a contagious disease.

DR. GARY SLUTKIN: For example, you're exposed to flu, you're more likely to get flu. You don't actually get flu without being exposed. Same thing for T.B., cholera and violence.

I mean, why does someone who was exposed to child abuse, abuse their own kids? That would be the person who you would think would be least likely to do it, because he knows how bad it was. But, in fact, he's picked up this contagious set of behaviors.

JOHN YANG: So Dr. Slutkin treats gun violence as a contagious disease. He founded Cure Violence, now an international effort that trains former gang members and felons to stop violence in its tracks, violence interrupters.

DR. GARY SLUTKIN: They are always in the community, aware of what's going on, and asking families and people, you know, who's upset? You know, who is — somebody slept with someone's girlfriend, someone was disrespected, someone owes somebody money. And we can reach those people with these health workers.

They may not look like health workers to everybody. They know how to cool people down, know how to buy time,

JOHN YANG: Chicago violence interrupter Chico Tillmon knows how to cool people down. He drove us around the South Side last week, where much of the violence happens.

CHICO TILLMON, Ceasefire Illinois: In early January, two cliques were arguing. So one clique went into another clique's neighborhood and got on Facebook Live and was like: F. you all. We're in your all neighbor. We're in your all gas station.

Within 30 minutes, on that walk from the gas station back to the house, two were dead, one was wounded.

JOHN YANG: It's not like this is a gang war over turf. This is just sort of…

CHICO TILLMON: Interpersonal. I said something you didn't agree with, you responded negatively, it ended up in gun violence.

JOHN YANG: Violence interrupter Ulysses "U.S." Floyd was a leader in one of Chicago's most notorious gangs, the Gangster Disciples.

ULYSSES "U.S." FLOYD, Ceasefire Illinois: I know I helped start this mess, so I wanted to help clean it up.

JOHN YANG: He told us that gangs are very different now than they were in his day.

ULYSSES "U.S." FLOYD: One or two men control everything. Now you have got a lot of different little gangs split all over. They are offsprings of the major gangs, what they call cliques. And they just do what they want to do. Ain't nobody really in control, no structure, no rules.

JOHN YANG: The number of neighborhoods where Chicago's branch of Cure Violence operates varies based on funding. But a Justice Department study found that, at one point, the group helped reduce violence by 40 percent to 70 percent in some of the areas where they were operating.

Today, they are in only five of the city's 77 neighborhoods. On our drive through the South Side, we saw children walking home from school.

What do you think when you look at kids that age?

CHICO TILLMON: I'm praying that they survive through this — through this epidemic that's going on in the city. It's not a Woodlawn problem. It's not a South Shore problem. It's everybody's problem. And we don't understand that until the disease hits home, until one of our loved ones is killed by gun violence. Then we want to get involved.

JOHN YANG: Police have seized 6,000 illegal guns this year. That's one every hour, many from nearby Indiana, where laws aren't as tough.

Chicago cops are feeling the heat, a federal probe of the use of deadly force and public trust at a breaking point after last year's release of a dash-cam video showing a white officer fatally shooting a black teenager 16 times.

After another deadly weekend, Superintendent Eddie Johnson virtually threw up his hands.

EDDIE JOHNSON, Superintendent, Chicago Police Department: It's not a police issue. It's a society issue. Impoverished neighborhoods, people without hope do these kinds of things. You show me a man that doesn't have hope, I show you one that is willing to pick up a gun and do anything with it.

LANCE WILLIAMS, Associate Professor, Northeastern Illinois University: I really see this problem as a cultural problem.

JOHN YANG: Lance Williams is an associate professor of urban affairs at Northeastern Illinois University and an inner-city youth advocate.

LANCE WILLIAMS: This is not a law enforcement problem. I mean, you can hire all of the police that you want. You're not going to solve this problem, because these young men are acting in alignment with their cultural value system. They need a cultural retooling process.

JOHN YANG: Williams says it's a culture that's developed in the absence of working institutions and in the midst of crushing poverty.

One big cause of much frustration? Nearly half of black men in Chicago aged 20 to 24 are not in school and out of work, far higher than the national rate of 32 percent.

LANCE WILLIAMS: There's a lot of rage. There's a lot of anger. They just see their lives, you know, just passing them by. They don't — they haven't been to school. They're not qualified for jobs. There are no viable businesses in their neighborhood, so they're really depressed. And then they're self-medicating through drinking and drugging. And the only individuals around them are other young African-American males like themselves who have these — these same forms of depression.

JOHN YANG: Another structural factor playing into the violence, Chicago is one of the nation's most segregated cities.

LANCE WILLIAMS: All of the poor blacks live way, way, way, way away from affluent people, from the business district, from the tourist district. You know, you have some kids in these neighborhoods far south that have never been downtown. Right?

And you have folks in the white communities who have never been to the South Side. So, what happens is, you have an out-of-sight, out-of-minute kind of deal.

CHICO TILLMON: I was 23 when I went to prison.

JOHN YANG: For Chico Tillmon, who spent 16 years and three months in federal prison, violence is never out of sight or mind, turning other people's lives around after turning his own around.

CHICO TILLMON: Being able to see all the violence and chaos in the community that I once was a part of, and that I once helped produce, pushed me or gave me an obligation to make a change.

JOHN YANG: Since you got out of prison…

CHICO TILLMON: Yes.

JOHN YANG:you got your bachelor's degree.

CHICO TILLMON: Yes, sir.

JOHN YANG: You got your master's degree.

CHICO TILLMON: Yes, sir.

JOHN YANG: You're working on your Ph.D.

CHICO TILLMON: Yes, sir.

JOHN YANG: How long, how many years are we talking about here?

CHICO TILLMON: Five years.

JOHN YANG: Pretty determined.

CHICO TILLMON: Yes.

JOHN YANG: Pretty motivated.

CHICO TILLMON: Yes, sir. I got out with a purpose, and I got out trying to not only do something that was beyond what I believed I could do, but to inspire hope within all the people that I left behind in prison.

JOHN YANG: Back on the corner of 75th and Stewart, Tamar Manasseh is also determined that change will happen.

TAMAR MANASSEH: It's going to take a lot of people all doing something, not saying something, but doing something to fix that problem. And the doing something is the sitting here. It's the sitting here, having a conversation.

I live on this block with you. I live in this city with you. I live in this country with you. And we're all affected by the same things. And, sometimes, when we don't talk to each other, when we don't interact, we miss that.

JOHN YANG: On one corner, a small effort in response to a big problem.

For the "PBS NewsHour," I'm John Yang in Chicago.

JUDY WOODRUFF: And online, you can take a closer look at Chicago's history of violence. We go back 50 years to count all of the city's homicide victims. That's at PBS.org/NewsHour.