Federal public health officials have issued a warning to summer swimmers regarding a trend of increasing outbreaks of cryptosporidium.

On Sunday, the Centers for Disease Control and Prevention (CDC) published a report on the increase in “crypto” outbreaks over the course of the last decade. Hundreds of outbreaks resulted in over 7,400 confirmed cases, tied predominantly to public swimming pools and water playgrounds.

Crypto are hardy parasites, which protect themselves with a shell that “allows it to survive outside the body for long periods of time and makes it very tolerant to chlorine disinfection,” according to information on the CDC website. Crypto parasites bring cryptosporidiosis, a disease that causes watery diarrhea that can last for weeks at a time.

As usual, the youngest and oldest among us are most vulnerable, as well as those with compromised immune systems, or those who may be in danger of “life-threatening malnutrition.”

Michele Hlavsa, head of the CDC’s Healthy Swimming Program, has warned that children are especially capable of, and vulnerable to, the spread of the dangerous parasite. “They don’t know how to use the toilet and wash their hands, or are just learning how,” she said. “But we as parents can take steps to help keep our kids healthy in the water, around animals, and in childcare.”

Hlavsa advised that children suffering from diarrhea should not be allowed to swim, and those swimming should avoid getting the water in their mouths. Because livestock can contribute to the spread of infection, people should also thoroughly wash their hands — with soap, not just hand sanitizer — after handling animals at zoos or county fairs.