THE CORONAVIRUS has shown the ability to transmit between people, cause serious illness and jump continents. These three conditions bring the world to the threshold of a pandemic. The word is scary, as Monday’s stock-market plunge reminded us, but most important is what to do about it. China might have blunted the virus with its draconian containment measures, but if containment fails, a host of problems will face every country, and few are ready.

The virus is highly infectious, and an explosion of cases over the weekend in South Korea, Italy and Iran suggests the window of opportunity for containment is “narrowing,” as the World Health Organization’s director-general, Tedros Adhanom Ghebreyesus, put it Friday. If the fire starts to burn beyond the firebreak, then strategy shifts to mitigation: helping the sick cope, saving lives, and keeping economic and logistical lifelines open.

So far, the impact in the United States has been small, with 53 cases as of Monday, but the Centers for Disease Control and Prevention says the potential public-health threat posed by the virus is “high” globally and to the United States. An early challenge if the outbreak produces hundreds or thousands of infections in the United States will be a sufficient supply of diagnostics to identify those who are ill; an early test kit by the CDC had flaws and hopefully will be improved and augmented with commercial test kits. China already has commercial test kits in wide use. A second challenge might be hospital capacity. A surge of thousands of patients would tax facilities everywhere, and plans must be laid for the flood. Also, health-care workers are at the front lines. In China, hundreds have been infected. It might be months or more before a vaccine or antiviral is available. To make sure workers are protected, health-care systems must prepare now with personal protective equipment — masks and more.

Just as important is public response. The virus understandably triggers fear. If the virus begins to spread widely, it will require such simple individual responses as hand-washing, and it may demand social isolation: closed schools, idled mass transit, disrupted businesses and canceled events. It is absolutely essential that governments — starting in Washington — be able to speak with credibility and authority. All experience suggests that truth and transparency are as indispensable as face masks. Once trust is broken, it is almost impossible to repair. Who will ensure trust at a time when political misinformation and disinformation run riot?

All of these risks multiply in countries without the existing health-care system and resources of the United States. One of the lessons of the Ebola outbreak in West Africa in 2014-2015 was to pay more attention to weak states. They stand to be overwhelmed.

If the case data from China on Monday is correct, and the epidemic there is easing, then there is hope it will not become a global pandemic. But a moment of optimism should not distract from preparations for a less favorable outcome.

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