From the WI Dept. of Natural Resources (http://www.dnr.state.wi.us/org/water/dwg/Crypto.htm):
"Treating drinking water using normal doses of disinfectants such as chlorine does not kill Cryptosporidium oocysts. Researchers have found it takes 90 minutes to kill 90 percent of Cryptosporidium oocysts in a water sample treated with 80 milligrams of chlorine per liter of water. Normally, chlorine is applied at about one milligram per liter. Stronger disinfectants, such as ozone, do a better job of killing these protozoans in a shorter time."

From the Center for Disease Control (http://www.cdc.gov/ncidod/dpd/parasites/cryptosporidiosis/factsht_cryptosporidiosis.htm):

"If you are unable to avoid using or drinking water that might be contaminated, then you can treat the water for Cryptosporidium by doing one of the following:
* Heat the water to a rolling boil for at least 1 minute. OR
* Use a filter that has an absolute pore size of 1 micron or smaller, or one that has been NSF rated for "cyst removal."

"Do not rely on chemicals to kill Cryptosporidium. Because it has a thick outer shell, this particular parasite is highly resistant to disinfectants such as chlorine and iodine."

p.s. The CDC says Giardia is also moderately chlorine-resistant but is killed or at least inactivated by boiling.

Sue