Close your leaky bubble.

Here’s the harsh reality of virus transmission: If someone in your family gets sick, the infection probably came from you, another family member or someone they know. The main way coronavirus is transmitted is through close contact with an infected person in an enclosed space.

“One of the challenges we have is that familiarity is seen as being a virus protector,” said Michael Osterholm, a member of President-elect Joseph R. Biden Jr.’s coronavirus advisory group and director of the Center for Infectious Disease Research and Policy at the University of Minnesota. “More likely than not, knowing someone is the risk factor for getting infected.”

This summer, 47 percent of Americans said they had formed a “pod” or social “bubble” that includes two or more households committed to strict precautions so the group can safely socialize indoors. But sometimes your bubble is leakier than you realize. Farhad Manjoo, an opinion columnist for The Times, had initially assumed his bubble was pretty small, but it turned out that he was having direct or indirect contact with more than 100 people.

Whether your bubble is just your immediate household — or you’ve formed a bubble with others — take some time to check in with everyone and seal the leaks. This requires everyone to be honest about the precautions they’re taking (or not taking). Dr. Osterholm said that convincing people that their friends might infect them has been one of the biggest challenges of the pandemic. He told the story of a man and a woman who both contracted Covid-19 after attending a wedding.

“He told me, ‘We didn’t fly. I knew everybody there,’” said Dr. Osterholm. “He somehow had the mistaken belief that by knowing the person, you won’t get infected from them. We’ve got to break through that concept.”

Mask up. You’re going to need it for a while.

A study by the Institute for Health Metrics and Evaluation at the University of Washington estimated that 130,000 lives could be saved by February if mask use became universal in the United States immediately.

Various studies have used machines puffing fine mists to show that high-quality masks can significantly reduce the spread of pathogens between people in conversation. And the common-sense evidence that masks work has become overwhelming. One well-known C.D.C. study showed that, even in a Springfield, Mo., hair salon where two stylists were infected, not one of the 139 customers whose hair they cut over the course of 10 days caught the disease. A city health order had required that both the stylists and the customers be masked.

Choose a mask with two or three layers that fits well and covers your face from the bridge of your nose to under your chin. “Something is better than nothing,” said Linsey Marr, professor of civil and environmental engineering at Virginia Tech and one of the world’s leading aerosol scientists. “Even the simplest cloth mask of one layer of material blocks half or more of aerosols we think are important to transmission.”