Most experts prefer iodine over chlorine. Iodine is less affected by pH or organic wastes and the taste is less offensive. After extensive testing, the U.S. Army now uses iodine in the field. Increasing one allows a reduction in the other. Other relevant factors are the temperature of the water and the presence of organic contaminants.
Low temperature slows reaction time requiring longer contact, especially to kill giardia.
Organic contaminants require increased concentration. Such contaminants are most commonly due to decomposing vegetation and are recognizable by a cloudy appearance in the water. Optimal concentrations of halogens readily kill bacteria, viruses, and most parasites. But one parasite, cryptosporidium, discovered in the mid-1980s, can withstand halogenation. Cryptosporidium causes diarrhea, cramps, and vomiting.