Beware, swimmers. Something may be lurking in the water, waiting to strike.

Cue the “Jaws” music.

Unlike the predator from the granddaddy of all shark dramas, this menace to swimmers has no fins. In fact, it can’t even be seen. Worse, it takes many forms — parasite, bacteria or virus — and can make you really sick.

In late summer, when sweltering heat drives many Minnesotans to seek relief in lakes and rivers, the risk of catching a bug from contaminated water rises. Because the lakes have gotten warmer along with everything else, there are more bacteria growing in them.

Swimming pools are OK, though, because there’s no algae growing there, right? Alas, it’s not that simple. All those people crowding into pools to escape the heat are bringing along germs with them. More people equals more germs.

“The reality is that when we’re swimming, we share the water with everyone. That means we share the germs, too,” said Trisha Robinson, Waterborne Diseases Unit Supervisor for the Minnesota Department of Health.

So what’s a poor, overheated, “pretty soon all the water is going to be frozen and we’ll be stuck inside for what seems like forever”-fearing Minnesotan to do?

For starters, keep your mouth shut. Most often, people get exposed to bugs by inadvertently swallowing water while swimming. If you want to be extra safe, use a nose plug and goggles.

And then relax, because you can still enjoy being in the water.

“Swimming is a great, healthy lifelong sport,” said Robinson. “We just want people to take some precautions to minimize the risk.”

Bugs bloom

A recent national study showed that swimmers are more likely than non-swimmers to come down with an illness that requires medical help after spending a day at the beach. Gastrointestinal and eye problems were the most common ailments, according to findings published in the Journal of Water and Health.

Late summer is peak season for parasitic and bacterial infections overall. The sustained warmer temperatures also create ideal conditions for harmful algal blooms to grow in lakes. It looks like pea soup or spilled green paint and has a swampy odor. It’s known as blue-green algae.

“Not all of them [the algae] contain the toxins that can make people or pets sick,” Robinson explained. “But if you see them, you should stay out, to be on the safe side.”

Lakes with low water levels are also places where bugs can fester.

Dr. David Romans, an emergency physician at Unity Hospital in Fridley, said drought conditions can make bodies of water stagnant over a longer period. That allows pathogens in the water to reach higher counts, he said.

Add to that the crowd factor at swimming holes this time of year and the risk of getting an infection rises. “Anytime you put population density in a very confined space, the potential for transmission of diseases increases.”

The most common source of germs in public swimming areas is the sick swimmer who refuses to stay out of the water. Symptoms of a water-related illness include diarrhea, fever and vomiting.

Kids tend to be especially vulnerable to exposure to germs, as they explore the water and tend to have a harder time avoiding ingesting some of it. Robinson recommends discouraging kids from behaviors that result in them getting more water in their mouth, and also telling them to spit it out and not swallow if it happens.

Minnesotans have become aware of the dangers lurking in lakes after a deadly brain-eating amoeba was deemed responsible for at least two child deaths in recent years. There were fears that a teenager contacted primary amebic meningoencephalitis (PAM) after swimming in Lake Minnewaska last month, but Minnesota Department of Health officials determined that the actual cause of death was a brain infection likely connected to an earlier head injury.

The amoeba that causes PAM is rare in Minnesota waters. Generally, it is found in warmer, southern states. Because that amoeba, like many others, enters the body through the nose, swimmers and divers can minimize risk by using nose clips or holding their nose shut. Robinson also recommends avoiding stirring up sediment at the bottom of shallow freshwater areas.

Pool party

Lakes aren’t the only places where germs live. Many people have a false sense of security about swimming pools because they believe that chlorine kills everything harmful, Robinson said.

“That’s simply not true. You still need to take precautions,” she said.

Cryptosporidium, or Crypto, is a parasite that can survive in a chlorinated pool for up to 10 days. It is a common source of gastrointestinal sickness.

Not all pools maintain the proper levels of chlorine. And pools tend to be more densely crowded than many lakes.

“That old adage that ‘dilution is the solution to pollution’ really is true,” Robinson said. “With rivers and lakes you have moving water or a larger volume of water” that thins out the concentration of alien substances.

Dr. Stacene Maroushek, a physician at Hennepin County Medical Center in Minneapolis, warns her patients about the dangers of infection from kiddie wading pools — a popular feature in many city parks. They’re often under-chlorinated, she said, and there are a lot of diapered kids using them.

One of the best prevention tools for water-related illnesses, as with all infectious diseases, is good hygiene. Showering before and after swimming can do much to keep the germs at bay.

“People who go to the lake or to the pool, they don’t want to come home with extra things,” Robinson said. “You’d like to come home with the memories — not the disease aspect of it.”