I know there were some bad cases of Bird Flu where some farm workers died. But has Bird Flu ever really spilled over and created some sort of fast spreading high fatality disease?

Also is it all just overblown like Covid 19 was. Why is there so much fear?

Is it really necessary to keep doing these chicken genocides every few years?

And can it even be stop if wild birds spread it and fly farm to farm? Why not just let it pass through get a herd immunity?