From tropical parasites to bacterial pathogens, here's what else might be swimming in the water.

It might just serve as a way to pass a rainy afternoon, but swimming may be one of humans' oldest hobbies. The earliest swimming pool dates back to 3000 BCE, in the Indus Valley.

Much later, in the 19th Century, swimming pools emerged in Britain and the US. But along with them came the challenge of keeping them hygienic. Even now, public – and private – swimming pools can become hotbeds of infection if they're not well maintained.

Swimming is considered highly beneficial for most people – providing a full-body workout and cardiovascular boost, while being low impact on the bones and joints. However, on rare occasions swimming pools have been linked with outbreaks of gastrointestinal and respiratory illness. Even in well-maintained pools, chlorine tends to be doing more to protect us than we might want to know.

So, just in time for the season of summer swimming (in the northern hemisphere, at least), here's what else might find its way into the pool water with you.

Over the last 25 years, swimming pools have been the most common setting for outbreaks of waterborne infectious intestinal disease in England and Wales. And the biggest culprit is Cryptosporidium.

This parasite can cause a stomach bug that can last for up to two weeks. People can experience diarrhoea, vomiting and abdominal pain – and around 40% will have a relapse of symptoms after the initial illness has resolved.

But most of the time, enteric diseases (the ones that cause diarrhoea and vomiting) clear up on their own in healthy people, says Jackie Knee, assistant professor in the London School of Tropical Medicine's Environmental Health Group. However, they can be a bigger concern for young children, the elderly and people who are immunocompromised, she adds.

Swimmers can catch cryptosporidium when an infected person has a faecal accident in the pool, or from swallowing residual faecal matter from their body, Knee says.

"And they could still shed [the parasite] afterwards, when they're not experiencing symptoms anymore," says Ian Young, associate professor at Toronto Metropolitan University's School of Occupational and Public Health in Canada.

You may go to many lengths to avoid swallowing pool water, but evidence suggests that some still ends up in our bodies.

One 2017 study conducted at public swimming pools in Ohio involved testing the blood of 549 people, including adults and children, after swimming in pool water for one hour. On average, the adults swallowed around 21 mL per hour, while children swallowed around 49 mL per hour.

When swallowed, the chances of this water being an infection risk differs depending on how busy the pool is. One study found that contracting cryptosporidium is more likely when swimming at peak times. The researchers tested water from six pools once a week for 10 weeks in summer 2017, and detected cryptosporidium in 20% of the pool samples, and at least once in each pool. Two thirds........

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