If part of a body is sick, the whole body can’t be healthy, and many cities across America have parts that aren’t doing very well. But there are regions that are trying to become healthier by coming together, rather than pulling apart. Tearing down a highway can be one way to do this. But it’s not the only way. My colleague Derek Thompson has written about the miracle of Minneapolis, where high-income communities share tax revenues and real estate with lower-income communities to spread prosperity. A year ago, I visited Louisville, where a court ordered the county and city to combine their school districts in order to integrate their schools. Today, Louisville is still trying to keep its county and city schools integrated, even after the Supreme Court told the city it no longer had to do so. In Chicago, a regional housing authority that covers eight counties, including Cook County, is working to move families from the inner city to higher-opportunity neighborhoods. Some cities use inclusive zoning, in which all new construction must include a certain percentage of housing for low-income residents, which means that the wealthy can’t separate themselves from the poor.