Why does the bird flu mostly occur in Asia?

Question:

I know that it's because its unsanitary conditions, but what else causes the spread of the bird flu.

Answers:
More people in Asia farm for a living so they have a closer proximity to birds. Why did it start there? I don't know, but historically that's where most pandemics starts. The Black Plague was brought to Europe in rat on a ship arriving in Italy from the orient.