Symptoms include watery diarrhea, stomach cramps, nausea and vomiting, possibly leading to dehydration.

“Parents can encourage their children not to swallow the water when swimming,” Hlavsa said, adding that swallowing even a single mouthful of contaminated water can cause illness. For this reason, parents should avoid buying pool toys that might encourage swallowing water, such as cups.

“Also, take kids on bathroom breaks every hour, and check diapers in a diaper-changing area and not right next to the pool,” Hlavsa said. “We all share the water we swim in, but we don’t want to share germs, pee or poop.”

Worldwide, cryptosporidium plagues water sources and water treatment systems, according to the World Health Organization, which notes that the pathogen may be increasingly contaminating food as well. Contaminated food, including raw milk and meat, farm-made apple cider, fermented milk, salads and raw vegetables, has caused outbreaks.

Crypto is a major contributor to infant and toddler diarrheal illness in seven countries in Africa and Asia. In children, it also can cause poor nutrition, which may be fatal. Primarily in developing nations, it may cause chronic diarrhea in people with HIV and other immunocompromised patients.

According to one report, the reported prevalence of cryptosporidium among patients with gastroenteritis is 1% to 4% in Europe and North America and 1% to 37% in Africa, Asia, Australia, and South and Central America.

Crypto may be found in water sources, including private wells, that have been contaminated with feces from infected humans or animals. Sewage overflows, polluted stormwater and agricultural runoff may also contaminate water.

In a single bowel movement, millions of cryptosporidium parasites can be released from an infected human or animal, according to the CDC.

Crypto cases in the US have approximately tripled since 2004, said the CDC, though it is unclear whether numbers are rising due to an increase in cases or more awareness and better detection tools.

Arizona identified 352 people sick with crypto during the 2016 swimming season, compared with no more than 62 cases for any one year from 2011 through 2015. Though Ohio saw no more than 571 crypto cases for any one year from 2012 through 2015, the state identified 1,940 in 2016.

“Crypto is extremely hard to kill with normal levels of chlorine, which is why it’s important to keep Crypto out of pools in the first place,” Hlavsa wrote. “We can all help do this by not swimming or letting our kids swim if sick with diarrhea.”

“Most people who have healthy immune systems will recover from Crypto without treatment,” she said, though if diarrhea persists for longer than three days, patients should contact their health care provider.

Generally, she said, backyard pools are less of a risk because fewer people swim in them. Still, even in properly treated water, crypto can survive up to 10 days.

However, the CDC does not recommend upping the levels of chlorine in a properly treated pool to try to avoid this problem. That might lead to something even worse.

Equipment failure and human error can lead to the release of toxic chlorine gas at public pools and water venues, as demonstrated by one alarming episode in California in June 2015.

About 50 people were in an outdoor Contra Costa County public pool when 34 began to vomit, cough or feel eye irritation, according to the new CDC report, also released Thursday. What happened? A chemical controller malfunction allowed sodium hypochlorite to react with muriatic acid, leading to the release of toxic chlorine gas.

Nationwide, more than 4,800 people visited emergency rooms for pool chemical-associated health issues in 2012, according to the report.

Chlorine poisoning symptoms depends on whether a person came into contact with the chemical or the gas, as well as amount and length of exposure, according to the CDC. Skin injuries among people who have a toxic exposure to liquid chlorine include burning pain, redness and blisters.

Chlorine gas was used as a chemical weapon during World War I. When it comes into contact with the eyes, throat and lungs, a damaging acid is produced.

Symptoms include blurred vision; a burning sensation in the nose, throat and eyes; coughing; difficulty breathing or shortness of breath; fluid in the lungs; nausea and vomiting; watery eyes and wheezing.

Half of the California bathers who became sick were treated at the scene. The others were transported to local ERs and were released that day or the next.

Hazmat personnel did not detect chlorine in the air when they conducted their check more than two hours after the initial 911 call. When the water was examined, measurements ranged slightly higher than California regulations for concentrations of free chlorine.

California records for 2008 through 2015 identified eight additional instances of toxic chlorine gas releases at public water venues caused by either equipment failure or human error. A total of 156 people had been sickened during that period.

To prevent these accidents, the CDC says pool chemicals need to be properly handled, stored and monitored. Pool operators and staff must be trained in pool chemical safety as well as appropriate operation and maintenance of equipment.

It is also important to follow standard pool policies, including evacuating bathers before a recirculation pump is restarted, as outlined in the CDC’s Model Aquatic Health Code.

State and local jurisdictions write, implement and enforce public water venue regulations, with no federal agencies overseeing facility design, construction, operation or maintenance.

The CDC developed the Model Aquatic Health Code, an ongoing collaboration among federal, state and local health officials, plus representatives from the aquatics sector.