"There is widespread concern among policymakers and public health experts about the possibility of a worldwide epidemic of avian influenza. Such pandemics are not new: there were three in the 20th century, of which one, the 1918"1919 Spanish flu outbreak, is estimated to have killed over 500,000 people in the U.S. and up to 50 million worldwide. Public health concerns arise because of the challenge of creating the public health infrastructure in the United States and other countries that would be adequate to meet the challenges of a severe pandemic. Although a pandemic could be caused by any of several influenza strains, scientists are particularly worried about H5N1, a strain that has caused repeated epidemics with high mortality among poultry in Asia, has spread from Southeast Asia to flocks in Central Asia and Europe, and has made the jump from birds to humans, causing the deaths of over 60 people. Moreover, viruses of the H5 subtype are not known to have ever circulated among the human population, which means that there would be little immunity to it. To date, close contact with infected poultry is thought to be required for human infection, but the danger exists that the virus will evolve in a way that allows for efficient human-to-human transmission. If the virus does acquire that capability, a worldwide epidemic, or pandemic, could occur. Depending on the virulence of the particular strain of flu, such an outbreak could have substantial consequences for people and economic activity around the world."
Congressional Budget Office: http://www.cbo.gov/