We were told by Park Rangers that giardia now lives happily in most US water sources. It has been spread by cattle and sheep who deposit the parasites everywhere, including around water sources. People drinking out of high mountain springs in the Sierras are sure to get the disease and it's no fun. Drink Coke and you'll be OK (except for a high caffeine and sugar load).
A few drops of Clorox will help. There are tablets for hikers and campers.