Whether you enjoy water aerobics or prefer to do laps solo, swimming is one of the best forms of exercise you can do. And with many pools opening for summer now, you might be feeling eager to get back in the water. But before you do, we wanted to warn you about some common germs that lurk in swimming pools and hot tubs, so that you’ll know what to do about them.

Chlorine works wonders to keep the water clean, but it’s not always perfect. Some pathogens can actually survive for some time in chlorinated water. Each year the CDC records data on outbreaks linked to public pools, with the following five pathogens being most common:

• • Cryptosporidium (a parasite that causes gastrointestinal illness)
  • Legionella (a bacteria which causes a severe type of pneumonia called Legionnaire’s disease)
  • Coli (a bacteria that can cause gastrointestinal illness)
  • Giardia (a parasite that causes severe diarrhea)
  • Norovirus (a virus that causes gastrointestinal illness)

If you swim regularly and develop signs of pneumonia, diarrhea, vomiting, and other concerning symptoms, make sure to mention your exercise activity when visiting your doctor. Those who smoke or are immune compromised are more likely to suffer severe lung infections from Legionella.

While swimming, protect yourself from infection by following these tips:

• • Don’t ever swallow the water
  • Watch out for red eyes and a strong chlorine smell; this often indicates an excessive level of urine, feces, and sweat in the pool
  • Avoid swimming if you have serious wounds, since the water can introduce pathogens into your body
  • Wear a swimmer’s cap or ear plugs to protect your ears
  • Shower and use plenty of soap right after swimming

And of course, do your part to protect others as well. Don’t go swimming if you’ve had diarrhea in the past two weeks. You can continue to shed pathogens long after your own symptoms have resolved, and potentially infect others.