WASHINGTON — Don't be alarmed if your local bank teller is looking a bit sickly over the next three weeks. It's only a cyber-illness.

Hundreds of banks and other financial institutions are participating in the largest test of its kind ever conducted to ensure the nation's financial system can keep functioning in case of an outbreak of pandemic flu.

The test began yesterday and is scheduled to run for three weeks. More than 2,700 financial institutions have signed up to participate, about five times the number the Treasury Department expected.

"This shows how much the business sector is focused on pandemic flu planning," Valerie Abend, Treasury's deputy assistant secretary for critical infrastructure protection, said.

Treasury, aided by other federal agencies and the private sector, has devised a three-week script for how a serious outbreak of bird flu might affect operations at banks, from the very biggest to the smallest, as well as at credit unions, securities firms and insurance companies.

The exercise also covers companies that provide critical processing to keep the flow of checks and money circulating around the country.

According to the doomsday scenario devised by Treasury, a number of cases of bird flu in humans are reported overseas and the illness spreads quickly to the United States by people traveling on international flights.

MULTIPLE CHALLENGES

From that beginning, the Treasury scenario presents financial institutions with a number of challenges over the course of the three-week exercise. The financial institutions got the first week's scenario over the weekend from an Internet site where the test is being conducted.

The whole exercise is part of a plan unveiled by President Bush in May 2006 directing various government agencies to upgrade their planning for pandemic outbreaks. The Government Accountability Office earlier this month criticized the administration for failing to conduct sufficient tests to make sure that the agencies understand their responsibilities.

One of the biggest challenges financial institutions will face is how to cope with absenteeism. In week one, the Treasury exercise directs the financial organizations to assume that 25 percent of their workforce is not coming to work, either because of illness or because of fear of being infected or because they are staying home to take care of children who can't go to school because the schools have closed.

To decide who is absent, the Treasury directs the institutions to assume that everyone whose last name begins with certain letters, which could cover the bank president down to the local teller, cannot come to work. The 25 percent absentee rate will jump to 49 percent in week two.

Abend said the various projections were compiled with the help of government scientists. Government financial regulators also helped put together scenarios on how the stock market will behave as well as what the value of the dollar and various commodities such as oil will be doing.

The dollar is projected to rise as investors seek a safe haven with the spreading global illness while stock prices are projected to fall because of worries about what the pandemic will do to economic activity.

CASCADING TROUBLES

Absent employees won't be the only troubles facing the financial institutions. Under Treasury's scenario, they also will have to cope with shrinking Internet bandwidths as more and more people try to work from home. Cash withdrawals from ATMs are expected to rise sharply, and getting the machines refilled will present problems because of rising absentee rates at the armored car companies and the difficulty of getting fuel for the armored trucks as gasoline refineries curtail their production.

By the end of the three weeks, Abend said, the government and the institutions participating will have a much better idea of just what a flu pandemic will mean in the United States. She said the test should get the institutions thinking about where they need to improve their plans.

"What would you do if you don't have access to key people? Have you cross-trained enough employees to sufficiently cover that?" she asked. "We want to do a really robust test."

Of the more than 2,700 organizations participating, two-thirds are banks, 20 percent are securities firms and 10 percent are insurance companies.