The infections occur when swimmers ingest water contaminated by diarrhea from a person infected by Cryptosporidium or Crypto, a parasite that is notoriously difficult to kill.

Crypto caused at least 32 outbreaks in swimming pools or water parks in 2016, compared with 16 in 2014, according to a report published in the U.S. Centers for Disease Control and Prevention's weekly report on death and disease.

Arizona last year reported that 352 people became sick with Cryptosporidiosis from July through October, compared with no more than 62 cases per year from 2011 to 2015. Ohio reported 1,940 infections in 2016, compared with no more than 571 in any one year from 2012 to 2015.

The CDC said it was not clear if there are actually more outbreaks, or if states are doing a better job of reporting them since it introduced a new DNA-based tracking tool in 2010.

Crypto is the most common cause of diarrhea outbreaks linked with swimming pools or water parks because it can survive up to 10 days in chlorinated water. It only takes a mouthful of contaminated water to make a healthy person sick for up to three weeks. Infections can cause watery diarrhea, stomach cramps, nausea or vomiting, and can lead to dehydration.

To kill the parasite, the CDC recommends closing pools and treating the water with high levels of chlorine, called hyperchlorination.

The CDC advises parents not to let children swim if they have diarrhea. People who are infected with Crypto should wait two weeks after the diarrhea stops before swimming. And to keep from getting sick, the CDC advises swimmers not to swallow pool water.