Cryptosporidium, or crypto, is a parasitic infection that causes diarrhea.

According to the US Centers for Disease and Prevention (CDC), the leading cause of diarrhea in the US is exposure to crypto-contaminated swimming pools.

The crypto parasite can survive for more than seven days in chlorinated pool water, ready to infect swimmers.

On Friday, the CDC reported that average annual crypto outbreaks are on the rise.

Visit INSIDER's homepage for more stories.

While rising summer temperatures may make diving into serene, cool public pools seem like a no-brainer, Americans may want to think twice before going swimming in pools or water parks.

That's because, according to the US Centers for Disease and Prevention (CDC), a parasitic infection known as Cryptosporidium, or crypto, is on the rise.

Between 2009 and 2017, 444 cryptosporidiosis outbreaks — which resulted in 7,465 cases of infection — were reported in 40 states and Puerto Rico.

The CDC reported that the number of reported outbreaks has increased an average of approximately 13% per year during that eight-year time period.

Read More: Terrifying facts to make you think twice before swimming in pools

Crypto-contaminated pool water is the leading cause of diarrhea outbreaks in the US, followed by contact with infected cattle livestock, and sick children at day care centers, between 2009 and 2017.

The parasite spreads when people swallow something that has come into contact with the feces of a sick person, like public pool water.

Crypto spreads very easily

It takes just 10 crypto parasites to get sick, and an infected person sheds 10 to 100 million in a single bowel movement, STAT reported. The parasite thrives in pools and water parks, because its tough outer shell makes it highly tolerant to chlorine.

According to the CDC, cryptosporidium can survive in a properly chlorinated pool for up to seven days. Exposure to treated recreational water in pools and water playgrounds was associated with 156 (35.1%) of the reported outbreaks between 2009 and 2017, resulting in 4,232 (56.7%) cases, the CDC reported.

The parasite is rarely fatal; only one death has been reported since 2009, according to the CDC. But 287 people were hospitalized between 2009 and 2017. In 1993, more than 400,000 people in the Milwaukee, Wisconsin area were infected when crypto got into the local water supply. It was the largest waterborne illness outbreak in US history.

Shutterstock/meunierd