Potential of pandemic
Avian flu, the H5N1 type especially, has the potential to become a dangerous human pandemic.
H5N1 has already caused epidemics among birds. The virus has been deadly to the birds, and has caused extremely high fatality rates.
In the past few years, the virus has shown that it can mutate so that it is able to infect humans. In the people that the avian flu has affected, it has caused very severe disease, and has proved to be fatal in more than 50% of cases. Obviously, this disease is much more deadly than regular influenza. If this disease becomes widespread in humans, there will be deadly results.
What H5N1 is missing is the ability to efficiently spread from human to human. There have been some cases of human to human transmission, but it has not gone beyond one person.
If the virus mutates (by antigenic drift, shift, etc.) so that it is able to spread among humans more efficiently, then avian flu could very likely become a deadly human pandemic.
This already has happened in the past. One example of an avian flu virus that had acquired the ability to transmit from human to human effectively was the Spanish flu virus, that caused the 1917- 1918 flu pandemic. The acquiring of the ability to spread quickly among humans produced deadly results - about 50 to 100 million people worldwide died in the pandemic.
It is hard to predict when, or if, the virus will make this mutation - it could be in a few years, or it could be tomorrow.