I'd like to caution against using iodine for water disinfection except for dire emergencies.

First, iodine is contraindicated for pregnant women and for children, which should tell you it's not harmless. It's also contraindicated for those with thyroid problems (which often are undiagnosed).

Second, as mentioned, iodine is ineffective against cryptosporidium (cysts, not eggs).

Third, a surprising number of people are allergic to iodine. I found this out after I had a severe reaction after only a month of using iodine to disinfect drinking water on weekend hikes. I developed a severe rash with deep lesions (lichen planus) which took months to heal and left permanent scars. This happened over 20 years ago. I still am so sensitive to it that one dish prepared with iodized salt or one serving of seafood is sufficient to make me break out. This is not trivial--most restaurants use iodized salt and I cannot accept an invitation to someone's house for dinner without asking that my food be specially prepared with non-iodized salt. Should I ever have to have a diagnostic procedure such as an angiogram that uses radioactive iodine--well, that's just too bad, I'll just have to drop dead from the undiagnosed ailment.

With chlorine dioxide and many varieties of filters available, there is no reason for anyone to use iodine. The risks are not worth it!