This is the second time in three years that the W.H.O. has designated an mpox epidemic as a global emergency. It previously did so in July 2022. That outbreak went on to affect nearly 100,000 people , primarily gay and bisexual men, in 116 countries, and killed about 200 people.

The threat this time is deadlier. Since the beginning of this year, the Democratic Republic of Congo alone has reported 15,600 mpox cases and 537 deaths. Those most at risk include women and children under 15.

“The detection and rapid spread of a new clade of mpox in eastern D.R.C., its detection in neighboring countries that had not previously reported mpox, and the potential for further spread within Africa and beyond is very worrying,” said Dr. Tedros Adhanom Ghebreyesus, the W.H.O.’s director general.

The outbreak has spread through 13 countries in Africa, including a few that had never reported mpox cases before. On Tuesday, the Africa Centers for Disease Control and Prevention declared a “public health emergency of continental security,” the first time the organization has taken that step since the African Union granted it the power to do so last year.

“It’s in the interests of the countries, of the continent and of the world to get our arms around this and stop transmission as soon as we can,” said Dr. Nicole Lurie, the executive director for preparedness and response at the Coalition for Epidemic Preparedness Innovations, a nonprofit that finances vaccine development.