Can iodine be used to disinfect water? I think it was Nat Geo.. Not sure exactly.. Where I heard, years ago, that a drop of iodine can be used to disinfect water of say, 1 litre. 

But isn't chlorine used to kill all bacteria? I mean I don't completely remember that documentary but I'm pretty sure it was  iodine that they used. 

Can iodine be used or does my memory fail me?
Short answer: Both chlorine and iodine in specific concentrations are used for water disinfection. 

Iodine has been used to disinfect water for a long time. It is used in concentrations between 2.5 ppm to 7 ppm, but there is some controversy regarding the maximum safe dose of iodine because excess of iodine consumption in humans can lead to various diseases(thyroid disorders being very common). I believe this is the reason we have moved towards more safer options like chlorine for water disinfection.