Transmissions of the human immunodeficiency virus have dramatically decreased over the last decade, but there are still widespread challenges to further reducing that number, according to the United Nations' annual report on HIV and AIDS released Monday.

Globally, new HIV infections are down by 33 percent since 2001 and have been more than halved among children. But HIV is far from being a problem of the past.

Last year 2.3 million people, including 260,000 children, contracted the virus. The pandemic is still especially prominent in sub-Saharan Africa, where 90 percent of the world's 3.3 million infected children live.

Much of the progress in combating HIV transmission has come from increased access to antiretroviral drugs, according to the report. They work by significantly reducing the amount of HIV active in infected people. If given to pregnant women, the drugs can prevent the transmission of HIV from mother to child.

The U.N. has focused on providing antiretroviral drugs to people in African countries most in need. In Ghana, for example, 90 percent of pregnant HIV-positive women had access to antiretroviral treatment last year, up from just 32 percent three years earlier. Such gains have prevented more than 670,000 children from contracting the virus from 2009 to 2012 alone, the report said.

Still, several barriers remain to further reducing the spread of HIV, including an increase in risky sexual behaviors in some countries.

"In several countries that have experienced significant declines in new HIV infections, disturbing signs have emerged of increases in sexual risk behaviors among young people," the report said.

But perhaps the biggest impediment is money.

Donations to the U.N. from member nations have remained flat since the financial crisis in 2008. That has left individual countries to pick up the tab, and the U.N. is struggling to come up with the amount it says it needs to reach its goal of providing 15 million people with HIV treatment by 2015.

Currently, $18.9 billion is budgeted globally for the U.N.'s HIV efforts, but UNAIDS director Michel Sidibé said the agency needs $22 billion to $24 billion annually by 2015 to meet its goals.

Earlier this year, Sidibé insisted that the investment would pay off, pointing out that "fewer deaths, [and] less sickness" takes a burden off health care systems and allows HIV-positive people to work and contribute to the economy for longer.

"If we do not pay now, we will pay later. We'll pay forever," he said.

Al Jazeera and wire services