Summer time is all about taking a dip in the pool, but it's no fun when it makes you sick.

Every year, thousands of Americans get sick from recreational water illnesses caused by germs spread from swallowing, breathing in mists or coming into contact with shared bodies of contaminated water. That water can be in back-yard swimming pools and hot tubs, public water parks and interactive fountains or in natural lakes, rivers, even oceans.

Diarrhea is the most common infection, but recreational water illnesses also can lead to skin, ear, respiratory, eye, neurological and wound infections.

"Children, pregnant women and those with weakened immune systems are most at risk for these illnesses," said Dr. Kevin Sherin, director of the Department of Health in Orange County, Fla.

Anyone who is ill should avoid swimming until their symptoms have passed, he added.

"Just one diarrheal incident can release enough germs into the water that swallowing a mouthful can cause diarrhea lasting up to two to three weeks," said Dr. Swannie Jett, health officer for the Department of Health in Seminole County, Fla.

A key to healthy summers starts with the back-yard swimming pool:

• To keep the water free of illnesses, check chlorine and pH levels before entering the water, county health departments advise. Even though chlorine isn't enough to keep pools illness-free, proper levels maximize germ-killing power.

• Don't swallow the water you swim in.

• Parents of small children should take youngsters on bathroom trips every hour and check diapers every 30 to 60 minutes.

The Centers for Disease Control and Prevention also recommends these guidelines for a healthy swim:

• Don't swim when you have diarrhea.

• Shower with soap before you start swimming.

• Take a rinse shower before you get back into the water.

• Take bathroom breaks every 60 minutes.

• Wash your hands after using the toilet or changing diapers.