Its strategy was not the full lockdown that China employed, or even the widespread restrictions that the U.S. and Europe have implemented. Instead, it focused on swift, widespread testing and contact tracing, our Interpreter columnist writes.

In the week after its first reported case, South Korea moved rapidly, eventually opening 600 testing centers and keeping health workers safe by minimizing contact. Once someone tested positive, officials meticulously traced their movements using security camera footage, credit card records, even GPS data from their cars and cellphones.

Reminder: South Korean officials caution that their successes are tentative. A risk of resurgence remains, particularly as epidemics continue raging beyond the country’s borders.

Here are the latest updates and maps of the pandemic.

In other developments:

Prime Minister Boris Johnson of Britain placed the country under a lockdown, banning people from leaving their homes for nonessential activities. He said the police would enforce the new rules.

India extended its lockdown to the general population, grounding all domestic flights. Its cases remain relatively low — 400 people — but the government wants to get ahead of the curve.

Chancellor Angela Merkel of Germany was tested for the virus after she learned that a doctor she saw was infected. Her first test came back negative, though she will keep taking tests and remain in isolation.

Prime Minister Shinzo Abe of Japan suggested that the Summer Olympics in Tokyo might need to be postponed, hours after Canada and Australia threatened to boycott the Games.

Nearly 70 drugs may be effective in treating the virus, researchers reported.

Markets: Wall Street fell on Monday even after the Federal Reserve unveiled an expansive new bond-buying program in the U.S. Major indexes in Germany, Britain and France were lower, and most Asian markets also closed down.

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