Swimming can be a great way to get exercise while escaping the summer heat.

But humans aren’t the only creatures that visit swimming pools, hot tubs, and water playgrounds.

Parasites, bacteria, and other pathogens can also thrive in recreational water.

According to a recent report from the Centers for Disease Control and Prevention (CDC), 493 recreational waterborne disease outbreaks were reported from 2000 to 2014 — causing at least 27,219 illnesses and eight deaths.

A third of those outbreaks were traced to hotel swimming pools or hot tubs.

Most were caused by Cryptosporidium, a chlorine-resistant parasite. It can cause diarrhea, vomiting, nausea, and stomach cramps.

Legionella and Pseudomonas were also common culprits of reported outbreaks. Legionella is a type of bacterium that causes Legionnaires’ disease. It results in flu-like symptoms and puts people at risk of severe pneumonia.

Pseudomonas is a common bacterium that can cause “hot tub rash” and “swimmer’s ear.”

To help protect swimmers from pathogens, proper pool maintenance is essential.

How to avoid getting sick after going for a dip

There are steps you can take to avoid getting sick at the pool. One tip is to only swim in well-maintained pools.

Some people are more likely to get sick from exposure to recreational water than others.

For example, certain health conditions and medications can weaken the immune system, leaving people vulnerable to infections.

Pregnant women, elderly people, and young children also tend to have weaker immune systems.

Children’s playfulness can put them at higher risk, too.

“Children, for example, tend to do more roughhousing when they swim, and they tend to swallow more water,” Kelly Reynolds, PhD, associate professor and program director of environmental and occupational health at the University of Arizona’s Mel and Enid Zuckerman College of Public Health, explained.

To help ward off illness, Reynolds encourages swimmers to try to avoid swallowing water from pools and other recreational facilities.

It’s also important to check facilities’ inspection scores, ask facility operators about their maintenance practices, and avoid facilities that aren’t properly maintained.

“The most important thing is to be sure the pool is chlorinated. If you do not smell chlorine, do not get into the pool,” Charles Gerba, PhD, a professor of microbiology and environmental sciences at the Mel and Enid Zuckerman College of Public Health, told Healthline.

“I also tend to go swimming in the morning and evening, when the pool is not too crowded,” Gerba added.

When more people are swimming, more chlorine is needed to maintain safe water chemistry.

To help protect swimmers, facility operators should check chlorine levels regularly, especially when the facility is busy.

It’s also important to maintain functional filtration systems and clean surfaces of slimy biofilms, where Legionella and Pseudomonas can flourish.

Avoid swimming while sick

Swimmers also have a role to play in protecting other community members from illness.

“If you have a gastrointestinal infection or diarrhea, then you shouldn’t swim in a swimming pool for at least two weeks after your symptoms have cleared,” Reynolds told Healthline.

“That’s a good recommendation to avoid spreading illnesses to other people,” she said.

People who are swimming with young children should also take regular bathroom breaks to help prevent accidental defecation in the pool or other water facility.

If a swimmer does defecate in the water, it’s important to tell the facility operator right away.

Many pools and other recreational facilities also advise guests to shower before they enter the water.

“They’re trying to get you to wash off any excess bacteria, or even skin oils and suntan lotion, which can tie up the chlorine that’s used in pools to kill the germs,” Reynolds explained.

“If you’re wearing sunscreen in the pool,” she continued, “you can actually bind the chlorine to your sunscreen, and it’s not available to kill the germs.”

Some pathogens are hard to kill

Even when facility operators maintain proper water chemistry, some pathogens are hard to kill once they enter recreational water.

“Unfortunately, Cryptosporidium is resistant to chlorine disinfectant, which is the primary chemical used to treat swimming pools,” Reynolds noted.

“It’s the number one cause of swimming pool infections right now,” she continued, “and that’s because it’s very difficult to treat and still be able to use the pool.”

In fact, Cryptosporidium can survive for more than seven days in water that’s maintained at CDC-recommended levels of chlorine concentration.

To effectively control it, facility operators need to filter recreational water using ultraviolet radiation or ozone systems.

They can also shock the water with high levels of chlorine to kill Cryptosporidium and other pathogens.

However, swimmers need to stay out of the water for extended periods of time while these procedures are underway.

That’s why it’s so important to take preventive steps to stop pathogens from entering the water in the first place.

“Again, the most important thing to control Cryptosporidium is that preventive action,” Reynolds said.

“You shouldn’t swim for two weeks after you’ve had diarrhea. If you’re ill at all, you really shouldn’t be swimming with other people. And showering is very important,” she added.