Avian Flu and Y2k

These two may both cause supply disruption. The difference as today CNN news point is quite interesting point of view.
Y2k: You know when it would happen, but you don't know what it costs.
Avian Flu: You know what it costs, but you don't know when it will happen.

Why we know what it costs? In the news, it is said that there is always three big Avian Flu in every century. So maybe people can estimate its cost from the history. But that cost is related how much effort people try to prevent or stop. Anyway, it is interesting criteria to divide different types of disruption, one time or recurrent?