For a century, doctors fighting infectious diseases have relied on a simple idea.

When you find a positive case, find the people that the patient might have exposed.

This is called contact tracing.

Contact tracing is hard enough with, say, HIV, where someone who tests positive might know the names and phone numbers of their recent sexual partners.

But with coronavirus, it's a different story.

The disease is airborne, contagious, and it can spread for days before a patient begins to show symptoms,

making it hard to figure out who might

have been exposed. That's what put the world on lockdown. Now, doctors say that contact tracing,

along with widespread, rapid testing, and social distancing is our only way out.

In the U.S., states and companies are trying to meet that need.

Massachusetts has hired a thousand people to start a contact tracing program, and Google

and Apple have collaborated on an app that uses Bluetooth to keep track of who you run into

all day, which, for the average New Yorker, might be as many as 75 people, not including contacts on public transit.

Today on the show, we're going to hear from a country where COVID-19 tracing has worked, and an American city that's about to try it.

I'm Henry Gorbar filling in for Lizzie O'Leary, and this is What Next TBD. Stay with us.

So what does it look like to spend a day in Seoul right now?

Honestly, it's business as usual.

This is Raphael Rashid. He's a journalist in Seoul.

It's like any other day, especially at the moment.

The worst is over, and right now I can do anything I want.

South Korea didn't flatten the curve. They never had any curve at all. On Wednesday,

they even had an election with record turnout. The shops are open, coffee shops are open,

...