The spread of the coronavirus is now a pandemic, officials at the World Health Organization said on Wednesday.

“We have rung the alarm bell loud and clear,” said Tedros Adhanom Ghebreyesus, the W.H.O.’s director-general.

Dr. Tedros called for countries to learn from one another’s successes, act in unison and help protect one another against a common threat.

“Find, isolate, test and treat every case, and trace every contact,” Dr. Tedros said. “Ready your hospitals. Protect and train your health care workers.”