This is grossly simplified, but it may help to shed some light. I pulled this directly from the EPA's website, specifically referring to Cryptosporidium and Giardia: "...studies concluded that chlorine dioxide is an excellent disinfectant against cysts and that chlorine dioxide is better than or equal to chlorine in terms of inactivation..." There's a lot more information to go over, but that's the gist of it.

Does that help?
Eric