How the coronavirus numbers changed so sharply

Officials in Hubei, the Chinese province that is the epicenter of the outbreak, said there had been more than 48,000 cases there, a 44 percent jump from the day before. They also reported 1,310 deaths, a 23 percent increase. The trend continued on Friday, though not as markedly, when Hubei officials disclosed another 4,800 cases and 116 more deaths.

Previously, cases were counted only if patients tested positive for the new virus. Now, the figures appear to include anyone who has a CT scan of the lungs showing a telltale pattern of pneumonia.

In the short run, the new accounting may deepen confusion about the true size of the epidemic. It also complicates efforts to track the growth of the disease over the last several weeks.

“We need to be careful in interpreting these numbers,” Dr. Michael Ryan, the World Health Organization’s emergency responses chief, said on Thursday. The larger number of cases, he said, “does not represent a significant change in the trajectory of the outbreak.”