(FORTUNE Magazine) - It's not the kind of milestone one wants to celebrate. On June 5, the 25th anniversary of the first published case of what was later identified as AIDS, an estimated 40 million people around the world will be carrying the human immunodeficiency virus that causes AIDS.

That's more than twice the number infected in 1995, despite the $8 billion spent fighting the disease last year. In sub-Saharan Africa, which has more than 60% of the world's HIV cases, only one in ten is getting anti-retroviral drugs. A recent UN study estimated that adequately coping with the epidemic will cost $15 billion this year and $22 billion by 2008.

But the full economic impact may not be felt until a generation from now. Roughly 1,800 children contract HIV each day, and the World Bank estimates that 20 million children--like this boy at a clinic in Nairobi--will be orphaned by 2010. World Bank economist Shanta Devarajan says AIDS is gutting Africa of its most active population, not only leading to a 50% drop in productivity growth but also starting a cycle in which parents can't educate their children. "The effects will be devastating in the long run," he says.

How bad could it get? South Africa averaged better than 4% growth last year, but Devarajan estimates its economy will shrink by half within 80 years.