We do NOT know that it is extremely lethal. We know that among REPORTED cases it has a high mortality rate. What we do NOT know is how many people haven't been hospitalized, or even reported it to medical authorities.

And it would require a fairly radical mutation to make it readily transmittable among humans. You almost have to go out of your way to catch HN51 at this time- the humans who have caught it have mostly had wounds or sores that were exposed while handling the carcasses of dead or dieing birds, or did not wear even the simplest mask while doing so. Those who wore protective equipment and showered thoroughly and decontaminated or destroyed their clothing afterwards have had almost none, zero, no infections. When you have had human-human transition, it is by communication of bodily fluids are being in very close proximity, such as a loved one acting as a caregiver. It's almost as hard to catch HN51 as most STDs.

I was on the state's Y2K team, and people on the state's Bird Flu working group know me personally and run things past me because they know about my "morbid little hobby". In both cases, the human factor is going to grossly outweigh the actual problem's direct effects.