With the virus now in more than 100 countries, the W.H.O. cited the “alarming levels of inaction” in declaring a pandemic, the first time it has used that designation since 2009. Although the term is largely symbolic, the decision to use it was a sign of growing concern among public health officials about the failure to contain the virus, which has infected more than 120,000 people and killed more than 4,300 around the world.

“Pandemic is not a word to use lightly or carelessly,” Dr. Tedros Adhanom Ghebreyesus, the head of the W.H.O., said at a news conference in Geneva. “We cannot say this loudly enough or clearly enough or often enough,” he added. “All countries can still change the course of this pandemic.”

As of late Wednesday, at least 1,240 people in 42 states and Washington, D.C., had tested positive for the coronavirus, according to a New York Times database, and at least 37 patients with the virus had died.

In New York, as in other world financial centers, already shaky markets fell another 5 percent to 6 percent, officially putting them into bear market territory for the first time in 11 years. In Washington, Dr. Fauci, the director of the National Institute of Allergy and Infectious Diseases, warned that the escalating outbreak should prompt organizations to scrap large gatherings.

“The bottom line: It is going to get worse,” he said at a congressional hearing. “We would recommend that there not be large crowds,” he added, citing as an example N.B.A. games.

Hours later, the N.B.A. announced that a member of the Utah Jazz tested positive for the coronavirus shortly before the tipoff of Wednesday night’s game against the Oklahoma City Thunder. The game was promptly canceled and the N.B.A. called off the rest of its games “until further notice,” adding, “The N.B.A. will use this hiatus to determine next steps for moving forward in regard to the coronavirus pandemic.”

Mr. Trump and other world leaders struggled to get ahead of the situation without a clear path forward. A day after he called for calm and assured Americans that the virus “will go away,” Mr. Trump signaled that the crisis was worse than he was saying by deciding to address the nation. But even then, he sought to minimize it as a “temporary moment of time.”