bluetooth and zigbee interference

all the wireless stated are under 2.4GHz frequency band, how can each of them avoid inteference from the rest or two wireless standard?

They dont avoid interference. If you place a Wi-fi (802.11), Bluetooth, Zigbee in close proximity they will interfere with each other. If you are using all 3 802.11 nonoverlapping channels along with Bluetooth and Zigbee you will experience performance degradation. It will work but at lower data rates......

BT 1.2 and above has AFH http://en.wikipedia.org/wiki/Bluetooth#Bluetooth_1.2 which improves resistance to radio frequency interference by avoiding the use of crowded frequencies in the hopping sequence.

WiFi has listen before transmit scheme that means it 1st listen to the channel and if it is not already occupied, it will transmit.

Hope this helped.