Heath officials in Arizona said Friday that more than 100 people have been sickened in an outbreak of diarrheal infection and that more than 20 water facilities may have been contaminated.

Maricopa County officials said that splash pads, water parks and public pools in the Phoenix area may have been contaminated with the pool-linked gastrointestinal illness cryptosporidiosis, or crypto, the Arizona Republic reported.

Dr. Rebecca Sunenshine, the medical director for the Maricopa County Department of Public Health, told the paper that there’s no reliable test for the disease in water, making the determination where the outbreak started difficult to find.

The microscopic, chlorine-resistant parasite that causes sickness is most commonly spread through water. Symptoms of the infection include diarrhea, vomiting and stomach pains, according to the Centers for Disease Control and Prevention (CDC). Crypto could be spread at streams, rivers, ponds and lakes as well.