Good mental health care is scarce in many parts of the United States, but it is nonexistent in most of the world. In developing countries, the ratio of mental health professionals to citizens is about one in a million — and that vast majority of people with treatable conditions like anxiety and depression are left to their own devices, or to the ministrations of local folk healers.

This week, the World Bank and the World Health Organization are convening hundreds of doctors, aid groups and government officials to start an ambitious effort to move mental health to the forefront of the international development agenda.

“The situation with mental health today is like H.I.V.-AIDS two decades ago,” Tim Evans, the senior director of health, nutrition and population at the World Bank Group, said Tuesday in a call with reporters. “We are kick-starting a similar movement for mental health, putting it squarely on the global agenda.”