When you look in a swimming pool, all you see is water. But what you don’t see might be making you sick.

“A lot of microorganisms can be in there that you won’t see without the use of a microscope,” said Dr. Bill Picking, who works in the division of molecular biosciences at Kansas University.

“Whenever you have groups of people in water, there’s always a chance of human pathogens being shed by some of the people.”

A swimming pool potentially can be riddled with bacteria, such as cryptosporidium and e-coli.

“There was an incident in Atlanta, where in a public pool, a child who had diarrhea from this organism (e-coli) actually shed these organisms into the water and a number of children actually became ill from this exposure,” Picking said.

Chlorine and other disinfectants keep most of the bacteria at bay, but unfortunately they don’t have an immediate effect.

“Once they are shed, there is a lag or time in which it takes to actually kill these organisms off,” Picking said. “During the process of killing them off, if someone were to ingest water or get some of the pool water in their mouth, they actually could contract diseases caused by these organisms.”

In some cases, the bacteria can lead to serious illness or even death. Picking said the possibility is “pretty remote, but real.”

“It’s very rare. With e-coli, for example, probably it would be more common as a food contaminant undercooked hamburgers, that sort of thing so it’s relatively rare that it would be contracted in a pool, but the possibility’s out there.”

There a few things you can do to protect yourself and keep pathogens out of the pool. Picking recommends showering before and after swimming and giving children ample bathroom breaks.

But overall, you shouldn’t be scared about splashing in the water this summer.

“It’s certainly not something that would drive me away from going down to the public pool,” he said.