UNITED NATIONS — The United Nations is scheduled to host a global meeting to stop the AIDS epidemic. Should groups that represent transgender people or drug users be allowed to participate?

No, according to a number of countries that belong to the 193-member General Assembly. And they have managed to block the participation of nearly two dozen organizations, from countries as diverse as Cameroon, Jamaica and Russia.

No reasons have been offered. None are required. The General Assembly operates by consensus, which means member countries can object to the participation of any nongovernmental organizations that are not accredited by the United Nations.

The blacklisted organizations and their supporters have loudly objected. Michel Sidibé, executive director of the Joint United Nations Program on H.I.V./AIDS, known as Unaids, warned in an email that “the list of excluded groups are many organizations that courageously and effectively speak to the needs of key population groups.”