The key to slowing down the spread of coronavirus infections is to have few, if any, close contacts. There’s just no getting around it.

The president has been infected with coronavirus. Schadenfreude is inappropriate. This is, however, a good opportunity for leaders to rethink their current policies and rhetoric on prevention — because what the president’s case highlights are the limitations of even the best testing regimes.

The president is tested for coronavirus every day. While the Centers for Disease Control and Prevention report the incubation period can be two to 14 days, it usually appears to be somewhere between three and five days after exposure. Given that President Trump tested positive on Thursday, he was most likely infected sometime between Saturday and Monday. We are usually concerned about a sick person being able to spread infections to others for up to two days before symptoms began or a test was positive. Therefore, the president might have been infectious to others as early as Tuesday. He probably interacted with many, many people in this time frame; likewise for Melania Trump and Hope Hicks.

The three of them will now need to isolate for at least 10 days. All of the people who were in close contact with them — and by close contact, I mean spent 15 minutes within six feet of them — will have to quarantine for two weeks, because they are at significantly increased risk of being infected themselves and infecting others. If any of them later test positive or show symptoms, they need to isolate for 10 days from that time.

This, in other words, is going to be a contact tracing fiasco — one that could easily shut down the White House.

Many, including me, have called for more testing for all. But what happened with Ms. Hicks and the president illustrates that testing doesn’t matter unless you close the infection loop with other interventions.

The president and the White House benefit from what is arguably the most rigorous coronavirus testing in the country. In the past, the White House has claimed that Mr. Trump is tested multiple times a day, and is the most tested man in the country.

Screening people without symptoms finds those who are infectious and gets them into quarantine and isolation earlier. But this doesn’t serve any purpose unless you’re also pairing this screening with careful behavior. Even if the infections of the president and others were discovered during regular screening, they still had a huge number of contacts; it’s still a disaster. If you’re going to lead a life where you could theoretically infect hundreds of people or more a day, slightly earlier knowledge doesn’t matter that much.

The most testing, the most advanced technology, and the best health care are simply not sufficient when it comes to this disease. What’s necessary are simple public health measures, like distancing, masking, washing hands and spending as little time as possible close together indoors in the same room. The key to slowing down the spread of coronavirus infections is to have few, if any, close contacts. There’s just no getting around it.

Unfortunately, this has not been the message coming out of the White House for some time. At the debate this week, the president ridiculed Joe Biden for wearing a mask too often. In remarks delivered Thursday night, Mr. Trump said that “the end of the pandemic is in sight.”

Many leaders at a variety of levels across the country have been following his lead in arguing that things are heading in the right direction, and that it’s perfectly fine to loosen restrictions. Politicians are meeting with large groups of people, indoors, with no masks and no respect for social distancing. Bars are opening. People gather for weddings and other occasions, acting as if there is no risk, the danger has passed, and everything is going to be OK.

They should seriously reconsider.

This is a disease where prevention matters most, and that’s where we need to sink our resources. It’s also what leaders need to promote, not ignore or debase.

It’s unsafe to hold crowded events. It’s unsafe to speak or shout without a mask, in indoor environments, for long periods of time. It’s unsafe to mingle, to hang out close together, and to gather in person when you could do so remotely. And this will still be the case for some time, even if we manage to scale up our testing regime significantly.

This is clearly not a time to celebrate or gloat. I hope everyone recovers quickly and easily. This news should serve as a reminder of how easily transmitted this disease is, how serious it can be, and how much education we still need to do to overcome misinformation. It is very difficult, and perhaps unwise, to regulate some behaviors. When that’s the case, it’s up to leaders to lead, both by words and examples.

[Aaron E. Carroll is a contributing opinion writer for The New York Times. He is a professor of pediatrics at Indiana University School of Medicine and the Regenstrief Institute who blogs on health research and policy at The Incidental Economist and makes videos at Healthcare Triage. @aaronecarroll]