As many as 25 percent of people infected with the new coronavirus may not show symptoms, the director of the Centers for Disease Control and Prevention warns — a startlingly high number that complicates efforts to predict the pandemic’s course and strategies to mitigate its spread.

In particular, the high level of symptom-free cases is leading the C.D.C. to consider broadening its guidelines on who should wear masks…

Researchers now say that people like Patient Z are not merely anecdotes. For example, as many as 18 percent of people infected with the virus on the Diamond Princess cruise ship never developed symptoms, according to one analysis. A team in Hong Kong suggests that from 20 to 40 percent of transmissions in China occurred before symptoms appeared.

The high level of covert spread may help explain why the novel coronavirus is the first virus that is not an influenza virus to set off a pandemic.