In a worst-case scenario, 70 million people, or 1 percent of those infected, could die from the virus that had spread beyond East Asia to Europe, the Middle East and Africa.
Developing countries would be the worst hit, with mortality rates double of those in high-income countries, the bank said.
"We estimate that a severe pandemic could now cost over 3 percent of the global economy's gross national product because of its impact on trade and economic activity," Jim Adams, head of the World Bank Avian Flu Taskforce, said.
"This could cost certainly over $1 trillion and perhaps as high as 2 trillion in the worst-case scenario," he told media persons. "So I think the economic threat remains real and substantial."
The World Bank announced a $15-million grant to Indonesia, the worst hit with 46 bird-flu deaths so far.
Fifty-five countries have reported outbreaks of the H5N1 virus, mostly since January.
About 80 percent of the 240 human cases and 141 deaths took place in Asia, the bank said.