The United States is winding up a particularly devastating week, one of the very worst since the coronavirus pandemic began nine months ago. On Friday, a national single-day record was set, with more than 226,000 new cases. It was one of many data points that illustrated the depth and spread of a virus that has killed more than 278,000 people in this country, more than the entire population of Lubbock, Texas, or Modesto, Calif., or Jersey City, N.J.

“It’s just an astonishing number,” said Caitlin Rivers, a senior scholar at the Johns Hopkins Center for Health Security. “We’re in the middle of this really severe wave and I think as we go through the day to day of this pandemic, it can be easy to lose sight of how massive and deep the tragedy is.” In California, where daily case reports have tripled in the last month, Gov. Gavin Newsom announced a new round of regional stay-at-home orders to address a mounting crisis over intensive-care beds. Some counties in the Bay Area said they were enacting tough new restrictions this weekend, before the state rules come into effect. And in South Florida, which is in the early stages of a new surge, physicians and politicians alike worried that there might not be enough resources to treat the sick.

As the virus has spread, infectious-disease experts have gained a better understanding of who among the nation’s nearly 330 million residents is the most vulnerable.

Nursing home deaths have consistently represented about 40 percent of the country’s Covid-19 deaths since midsummer, even as facilities kept visitors away and took other precautions and as the share of infections related to long-term care facilities fell substantially.

Underlying conditions have played a pivotal role in determining who survives the virus. Americans who have conditions like diabetes, hypertension and obesity — about 45 percent of the population — are more vulnerable

.And new evidence has emerged that people in lower-income neighborhoods experienced higher exposure risk to the virus because of their need to work outside the home.