My impression about cryptosporidia oocysts is that they come close to being everywhere on the typical dairy farm. Tough to avoid exposure. 
In a recent research report on dairy calves [ T.M. Hill and Others, "Effect of feeding rate of milk replacer with early weaning and protocols for water treatment and sanitation on dairy calf growth and health." The Professional Animal Scientist, 31 (July 2015) 375-382] the calves with abnormal manure were checked for parasites. 
They had 48 bull calves on the feeding trial, all of them came from one dairy farm. Of these 22 had abnormal feces (46%). This diarrhea rate may have been expected given that the blood serum total protein levels averaged just under 5.0.
What percentage of these calves had cryptosporidia in their feces? ONE HUNDRED PERCENT!
This high rate of parasite presence was in spite of exceptional efforts at the research facility to prevent inoculation from water and feeding equipment. My best guess is that the calves had already been exposed to an infective dose of crypto oocysts before they left the host farm. You will recall that this parasite is autoinfective (that is, the oocysts do not have to leave the body in order to re-enter the g.i. tissues) so no additional exposure would be needed to reach a clinical level of infection. 
For a one-page summary about cryptosporidia click HERE
For a protocol for using chlorine dioxide for crypto control click HERE