Most viruses seem to attenuate when moving form one host species to another, but not always.

If H5N1 became easily tranmissable between humans and retained it's current 50% mortality rate, it would be a disaster of unimaginable proportions.

The "Spanish Flu" of 1918-19 had a mortality rate estimated at 2.5% and it killed somewhere between 20 and 40 million people, of a world population of about 1.8 billion. This would imply that between 45% and 90% of all humans in the world were infected.

Can you imagine 90% of today's world population being infected by an agent that was 50% fatal?

The effect of the influenza epidemic was so severe that the average life span in the US was depressed by 10 years. The influenza virus had a profound virulence, with a mortality rate at 2.5% compared to the previous influenza epidemics, which were less than 0.1%. The death rate for 15 to 34-year-olds of influenza and pneumonia were 20 times higher in 1918 than in previous years (Taubenberger). People were struck with illness on the street and died rapid deaths. One anectode shared of 1918 was of four women playing bridge together late into the night. Overnight, three of the women died from influenza (Hoagg). Others told stories of people on their way to work suddenly developing the flu and dying within hours (Henig). One physician writes that patients with seemingly ordinary influenza would rapidly "develop the most viscous type of pneumonia that has ever been seen" and later when cyanosis appeared in the patients, "it is simply a struggle for air until they suffocate," (Grist, 1979). Another physician recalls that the influenza patients "died struggling to clear their airways of a blood-tinged froth that sometimes gushed from their nose and mouth," (Starr, 1976). The physicians of the time were helpless against this powerful agent of influenza. In 1918 children would skip rope to the rhyme (Crawford):

I had a little bird,
Its name was Enza.
I opened the window,
And in-flu-enza.

From Stanford

The skipping rhyme is especially striking in view of the carriers of H5N1.

Medicine has changed a lot since 1918, and methods for recognizing and handling pandemics have improved, but on the other hand populations are a lot more concentrated and human mobility has vastly increased.