Microbes, including bacteria that live on our skin and in our noses, get dropped into the pool whenever we take a dip. Not all of them make us sick, and chlorine can kill germs, but sometimes it takes days for that to happen. And if your pool smells like chemicals, that’s not a sign that it’s clean. In fact, the stronger the odor, the harder the chlorine is working to eliminate things like urine, sweat, dirt, poop and other things that swimmers trail in.

Here’s a handy (and slightly scary) rundown of the bugs and other things that every swimmer brings into the pool, from the Centers for Disease Control. Their advice? Take a dip and enjoy your swim. But shower before you jump in. And don’t drink the water.

Contact us at letters@time.com.