The number of new cases of tuberculosis in the world each year is falling for the first time in at least two decades, marking a long-sought turning point for the globe’s second-biggest infectious killer.

A lung infection that can spread to virtually any organ, TB has afflicted people for at least 15,000 years. Last year, it killed 1.4 million people. Among infectious diseases, only AIDS took a larger toll.

Tuberculosis has always been an infection closely linked to poverty, but its spread saw an unexpected boost 25 years ago with the emergence of the AIDS epidemic. People infected with HIV are 30 times more likely to develop TB than those who are not, and one-quarter of those who died from TB last year had the AIDS virus.

The downturn in new cases, announced Tuesday in Washington by World Health Organization epidemiologists, began in 2006 but wasn’t detected until this year, when new data from China, India and 17 African countries became available. That allowed the revision of global case estimates — and ultimately of the disease’s entire trend.

“We are seeing a significant benchmark in the control of the epidemic. This is a sign that the commitment and investment in TB control has paid off,” said Mario Raviglione, head of WHO’s Stop TB Department.

The decline in new cases is the product of many forces.

The peaking of the AIDS epidemic in Africa has slowed the spread of TB there at the same time that infusions of foreign aid, much from the United States, have helped countries find and treat infected people. TB mortality in China fell by almost 80 percent between 1990 and 2010, and the rate of new infections dropped about 3 percent a year. Several other countries with high infection rates — including Brazil, Cambodia and Uganda — cut TB mortality in half during that period.

In all, there were 8.8 million new cases of tuberculosis in 2010. About 59 percent were in Asia and 26 percent in Africa. Only 3 percent occurred in the Americas: Latin America has seen major reductions in recent years.

Of the 12 million people living with active tuberculosis last year, 650,000 were infected with multiple drug-resistant strains of the TB bacterium, which makes a cure more expensive and less likely. The overall cure rate for treated new cases of TB was about 87 percent.

WHO officials chose to announce the new numbers in Washington rather than at the organization’s Geneva headquarters in hopes of encouraging U.S. lawmakers to spare global TB control funds from the current round of budget-cutting.

“This momentum could be turned into complacency. Without the U.S. government’s investment, we would have stagnation for the next few years,” Raviglione said.

The U.S. federal government spends about $375 million on efforts to treat and control TB in developing countries: $225 million through the U.S. Agency for International Development and the rest through the President’s Emergency Plan for AIDS Relief.

The Senate version of the 2012 budget roughly keeps overseas TB spending at current levels. The House version cuts the Obama administration’s $7.83 billion “Global Health Initiative” by $700 million but doesn’t specify which programs should be cut. Most observers believe that TB wouldn’t be spared.

Should the congressional “supercommittee” fail to reach a deficit-reduction agreement, even larger cuts would likely follow.

The U.S. contribution is viewed as critical even though, unlike the case of AIDS, many developing countries manage to pay for TB control and treatment out of their own budgets. Overall, domestic funding accounts for 86 percent of all TB spending. The remainder is made up by foreign assistance, more than three-quarters of which comes through the Global Fund to Fight AIDS, Tuberculosis and Malaria, a 10-year-old organization to which the U.S. government is the largest contributor.

Representatives of other public health organizations at the announcement also called for continued efforts at TB control, even in rich countries where the infection is rare.

“The United States really dropped the ball in the late 1980s and early 1990s, and we paid the price with a resurgence of this deadly and costly disease,” said Harold Jaffe, a doctor and epidemiologist at the federal Centers for Disease Control and Prevention.

After years of neglect, research and development is slowly adding new technology to TB control, including a rapid way to identify drug-resistant strains of the TB microbe.

There are 10 “candidate” vaccines for TB in the research pipeline and two new drugs that could be available as soon as 2013.

“The field of research on TB has really missed generations of advances in biology and technology. We really need to bring TB research from the 19th into the 21st century,” said Anthony S. Fauci, director of the National Institute of Allergy and Infectious Diseases, who also spoke at the media event.

A key unanswered question, he said, involves the millions of people who become infected in childhood and whose immune systems successfully suppress the disease. About 10 percent eventually develop active TB, often in old age. If they could be identified early, many new cases could be prevented.