Global bank HSBC believes that as many as half its staff could be knocked out of action by a bird flu pandemic.
The worry is that the flu will spread globally and from birds to humans
HSBC said that while this was a "worst case" scenario, it was common sense to plan for every eventuality.
The one-in-two rate is higher than many official forecasts, including the 25% seen by the World Health Organization.
Concerns about the spread of bird flu have increased after the virus claimed its first victims in Turkey. HSBC has 253,000 staff globally in 77 countries.
Its business in Asia was affected by the outbreak of the Sars virus in 2003.
A spokeswoman for the lender said that it routinely planned for events that may pose a threat to its staff, customers and business.
"While we have no reason to disbelieve the World Health Organization guidelines on bird flu, it is entirely proper that we prepare for any contingency," HSBC said.
The bank's spokeswoman added that the 50% total included staff who might be unable to travel or needed to take time off to care for loved ones.
The bank's plans were first reported in the Financial Times (FT) newspaper.
"None of us know the virulence of the [bird flu] virus, but I would rather be prepared for the worst," Bob Piggott, the bank's head of crisis management, told the FT.
Mr Piggott said that HSBC was preparing for staff to work from home, or via video link and teleconference facilities.
It also is planned to clean offices once an hour in an attempt to limit the possibility of infection, something that was done in HSBC's offices during the outbreak of Sars.
Mr Piggott added that while many of the staff would be ill for at least a week, others would be caring for loved ones and family members, or avoiding public transport and public places.
He said that several other banks were working with similar estimates.
The comments follow a warning in a Swedish study that the number of cases of bird flu in humans may have been hugely under-reported.
At the same time, Turkey is increasing its efforts to contain an outbreak of the H5N1 strain of bird flu, after the virus killed at least two people and infected 14.
A bird cull has been implemented, although authorities have been hampered by cold weather and the remote location of many villages.
According to the WHO, the H5N1 strain of bird flu is well-established in the region.
However, there is no sign the virus is passing from human to human, without which a pandemic is impossible.