Polk County health officials are urging swimmers to use caution to defend against a waterborne illness.

Officials tell Des Moines television station KCCI that they have logged 68 cases of cryptosporidium this July, up from six cases last July. Cryptosporidium is a chlorine-resistant, highly infectious parasite that causes diarrhea and abdominal cramping.

Signs have gone up at some Des Moines-area aquatic centers warning people with diarrhea to stay away.