The IMF, a leading global financial watchdog, said a pandemic would trigger sharp asset-price falls, a slump in tourism and trade and lead to mass absenteeism from work.
The warning came as Germany said it was testing seven ducks in Bavaria for the virus, in what could the country's first case in domestic fowl.
The IMF said business continuity planning would become a critical component in preventing a crisis in the financial sector.
"However, in many countries, business continuity planning has not yet addressed the specific risks arising from a pandemic, particularly from possible high absenteeism," it said. "Perhaps because an avian flu pandemic may appear to be a low-probability event, many countries are only starting to develop a comprehensive approach to this threat."
However, it said countries with experience of Sars, and those with large, complex financial systems such as the UK, were well-prepared.
The IMF said its staff had started discussions with central banks, regulators and financial institutions to find out how prepared they were.
Last year the World Bank, the sister organisation to the IMF, said an epidemic could wipe $800bn (£460bn) off world economic growth.
The IMF said the main impact would also include disruptions to transport, payment systems and public utilities that would expose financial vulnerable business to the "risk of bankruptcy". The organisation said: "Moreover demand could contract sharply, with consumer spending falling and investment being put on hold. Financial repercussions could further exacerbate the economic impact."
This would put governments' finances under "substantial pressure", the IMF said, as it increased spending on health, public safety and social welfare at the same time its tax revenues would begin to dry up.
It urged central banks to ensure there was adequate supply of cash in the economy while regulators should refrain from enforcing capital-adequacy rules that would be broken as asset prices plunged. "Market operations could become more disorderly in the case of a breakdown in the trading infrastructure, leading to limited or intermittent trading," it said.
However, the report added: "Once the pandemic has run its course, economic activity should recover relatively quickly."
The World Health Organisation said 176 people have been infected with bird flu around the world since 2003, and 98 have died.