Gone are the days when Americans could easily understand the virus by tracking rising case numbers back to discrete sources — the crowded factory, the troubled nursing home, the rowdy bar. Now, there are so many cases, in so many places, that many people are coming to a frightening conclusion: They have no idea where the virus is spreading.

“It’s just kind of everywhere,” said Crystal Watson, a senior scholar at the Center for Health Security at the Johns Hopkins Bloomberg School of Public Health, who estimated that tracing coronavirus cases becomes difficult once the virus spreads to more than 10 cases per 100,000 people.

In some of the hardest-hit spots in the United States, the virus is spreading at 10 to 20 times that rate, and even health officials have all but given up trying to figure out who is giving the virus to whom…

“I was so careful,” said Denny Taylor, 45, who said he had taken exacting precautions — wearing a mask, getting groceries delivered — before he became the first in his family and among his co-workers to test positive for the virus. Lying in a hospital bed in Omaha this past week, he said he still had no idea where he caught it.