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More than 100 people in Maricopa County, Arizona, have been infected with Cryptosporidium, a parasite officials believe was in at least 20 pools in the community.

Cryptosporidium, or "Crypto," causes problems ranging from stomach cramps to vomiting and fever. The most common symptom is watery diarrhea, which generally begins a week after infection. Those with healthy immune systems don't need treatment and usually recover after a week or two.

It's passed through stool and is typically transmitted through contaminated drinking water or recreational water facilities that have infected fecal matter in them.

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The highly contagious parasite is difficult to get rid of because chlorine doesn't always kill it, according to the Mayo Clinic. Alcohol-based hand sanitizer isn't effective against it, so washing hands with soap and water is considered the best prevention.

Maricopa County health officials said those infected have identified more than 20 water facilities where they believe they may have picked up the disease.

Scanning electromicrograph of Cryptosporidium Science Picture Co. / Getty Images\

“Unfortunately, there is no reliable test for Crypto in water, so there is no way to know which pools are contaminated with this parasite,” Dr. Rebecca Sunenshine, medical director for Maricopa County Department of Public Health, said in a press release. “The most important thing the public can do to prevent spread of this disease is to stay out of the water if you have diarrhea, until at least two weeks after symptoms resolve.”

She advised swimmers to avoid swallowing water from pools and to seek medical care if symptoms persist.

“If you have diarrhea lasting longer than 10 days, blood in your stools, or have trouble staying hydrated, see a healthcare provider and let them know you may have been exposed to Cryptosporidium," she said.

Officials didn't say which pools had been identified but said the Maricopa County Department of Public Health notified the facilities and recommended they follow decontamination guidelines from the Centers for Disease Control and Prevention.

The parasite was first discovered in Maricopa County, which encompasses Phoenix, on Aug. 4 after 19 cases were reported in July. Last July, there had only been four cases, according to NBC affiliate KPNX.