"The 20% fatalities . . . will it be the elderly and infirm or what are the "risk factors" likely to be?"
Ben, I'm going to have to let TC answer your precise question. What's clear is that "bird flu" has killed many young, and not so young, but healthy people who have contracted it. Like the other poster says about her grandfather, in the 1918 epidemic, people were in perfectly good health in the morning and dead by nightfall. So these virulent viruses can really strike a person down.
Interestingly, it seems to be affecting younger people.
In Turkey (where I got that 20% figure from), only 2 people out of the 20 cases there were older than 18.
Some are saying it's because the kids are more likely to be playing in the chicken coops, or interacting with the chickens, or they're less sanitary.
Others say it might favor them for some reason or another. We really wouldn't know unless a pandemic actually happened.