I found this
tidbit on Webmd.com:
The H5N1 bird flu bug has been particularly deadly for people unlucky enough to catch it from poultry. But if the bug learns to spread among humans, it almost certainly won't be as deadly as it is now, says Ira Longini, PhD, professor of biostatistics at Emory University's Rollins School of Public Health.
"Avian H5N1 looks like a 70% case fatality in humans. But this has never been true of any human strain," Longini told WebMD last December. "There has never been any human influenza virus that has behaved that way in recorded or even unrecorded history. The case fatality of even highly virulent flu strains are a couple of deaths per 10,000 people."