A large percentage - some surveys get around 25 percent- of AT thruhikers do not treat their water at all and another 10 to 15 percent use only mild treatments such as household bleach or iodine tablets that are not considered effective against giardia cysts. Others treat "as needed" which means when the water is suspect for some unspecified, probably subjective, reason. The incidence of true giardia on the AT is low regardless of treatment method, although hikers often call common tourista "giardia" not knowing the difference. One may assume they have been brainwashed. Me, I use bleach when the watershed of a source seems open to contamination. No trouble in 6 trail months over the last 11 months.