Hi everyone,
So I know a lot of us carry iodine for water purification. I did a bit of research today, and found that BOTH iodine AND chlorine are pretty poor at purifying water from giardia or cryptosporidia.

Regarding cryptosporidia:
"For individual use, water can be decontaminated by bringing it to a boil or by passing it through a filter with a 1-μm or smaller pore size. Generally, chlorine and iodine are ineffective."

Regarding giardia:
"Individual water supplies, such as for travelers or backpackers, should be boiled or filtered. Water filters with pores of 1 μm also may be used. Giardia and Cryptosporidium are relatively resistant to chlorine or iodine treatment."

This article came from the journal "Gastroenterology Clinics" Vol 35, No. 2, June 2006. An interesting read if you know your science, though it's pretty basic.