Infections are not ever CAUSED by dehydration! They are caused by bacteria.
Yes, if one is dehydrated, it makes it easier for the unwanted bacteria to "set up shop", but this is generally not going to have anything with a the fiber in a Bowel Cleanse (especially the small amount in Colonix).
Dehydration occurs most frequently (when all systems are basically healthy) due to not drinking enough fluids, especially pure water.