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It’s shark season — primarily on cable TV. As the decades-long tradition of “Shark Week” approaches, you can expect once again to hear of “serial killer” sharks, attacks near major coastal cities, and menacing, massive shark swarms.

But, as you probably also know, shark attacks are incredibly unlikely. You’re 75 times more likely to be killed by lightning than by a shark. On average, one person dies of a shark attack every other year in the United States.


But just when you thought it was safe to go back into the water … just kidding, it’s still mostly safe. But swimmers face real risks that are of a much less telegenic variety.

Here, seven things swimmers should actually be afraid of — and how to avoid them.

1. Cryptosporidium

Outbreaks of this parasite, known as crypto, have doubled over the past couple years. Crypto spreads through the feces of infected animals, including humans — a running theme of this list. It takes just 10 crypto parasites to get sick, and an infected person sheds 10 to 100 million in a single bowel movement. Crypto is the most common infection at recreational water sites like pools and water parks, since the parasite’s tough outer shell makes it resistant to chlorine.


Outbreaks are typically small, but, in 1993, over 400,000 people in the Milwaukee area got infected when crypto entered the local water supply — the largest waterborne illness outbreak in U.S. history.

2. Pseudomonas

After reading about crypto, you might be tempted to take a dip in hot water to ward off microbes. After all, boiling water is a common sterilization technique. But the average hot tub is 102 degrees F — not hot enough to kill germs, but warm enough to break down chlorine. This makes hot tubs harder to disinfect.

One of the most common infections for hot tubbers is pseudomonas, a rod-shaped bacterium that burrows into hair follicles and causes “hot tub rash.” Just days after a relaxing summertime dip, you may notice red, itchy bumps across your skin, especially in areas covered by your swimsuit. A 2011-2012 study found that all six pseudomonas outbreaks reported to the Centers for Disease Control and Prevention occurred in hot tubs. The bacteria can also infect the ear canal, leading to the painful and itchy “swimmer’s ear” that children often get.

3. Shigella 

Let’s get back to poop. Shigella — which includes four different types of bacteria — causes 500,000 cases of diarrhea in the United States each year. Other symptoms include fever, abdominal pain, and the nagging feeling that you need to go to the bathroom. These symptoms usually start a couple days after swallowing a bit of contaminated water — usually at a lake or pond, but also at beaches. In most cases, rest and fluids do the trick, but antibiotic-resistant shigella strains are also out there, making some cases require heavy-duty treatment.

4. Legionella

Unlike other members of this list, legionella infects your airways and not your gut. Hot tubs — with that warm, steamy air we all just love — are a common place to get infected. Infection leads to one of two illnesses: Legionnaires’ disease or Pontiac fever. Legionnaires’ is a form of pneumonia that tends to affect smokers, those over 50, and others with weak lungs or immune systems. The disease is serious, leading to severe coughing, fever, and shortness of breath, and requires antibiotic treatment. Pontiac fever is milder, and leads to a fever and muscle aches that usually subside without treatment. Only 5 percent of those exposed to legionella develop Legionnaire’s, while over 90 percent develop Pontiac fever. 

5. Norovirus

This virus, shed through stool (of course), infects and causes inflammation in the stomach and intestines. Norovirus is sometimes mistakenly called the “stomach flu,” but is not related to influenza. As few as 18 microscopic viruses are enough to get you sick, leading to a bad case of diarrhea and vomiting that is sure to keep you away from the water. Seventy swimmers at a lake in Oregon found that out for themselves when they got infected during the summer of 2015.

There’s no vaccine for norovirus — not yet, anyway. Scientists are actively working on that, and, last year, a group of scientists at Baylor College in Texas figured out how to make the virus infect human cells in a dish. Researchers have tried — and failed — to do this for decades, and the feat may lead to a better understanding of how the virus works and how to stop it.

6. Cyanobacteria

These blue-green bacteria thrive during the summer — sometimes to a dangerous extent. Runoff of fertilizers promotes explosive growth of the bacteria, which coat beaches, lakes, and rivers like a thick pea soup. These bacterial blooms can also contain toxins that are harmful on contact, irritating the skin, eyes, and throats of swimmers. And, if you swallow any of the water, you can suffer from headaches, vomiting, and, in severe cases, liver damage. Currently, there are no specific treatments for cyanobacteria toxins, but doctors can manage symptoms as a patient recovers.

In 2016, over 100 people got sick from a toxic cyanobacteria bloom at Utah Lake during a summer heat wave. With rising temperatures, outbreaks may become more common; the National Ocean Service reports that harmful blooms are on the rise along the coasts.

7. Naegleria fowleri

This parasite is known as the “brain-eating amoeba.” Do we need to say anything more? These amoebas lives in warm bodies of fresh water and infect you when you get contaminated water up your nose. The parasite travels into your brain and triggers a deadly inflammatory response about five days after exposure. The result: an excruciating headache and fever spiral into seizures, hallucinations, and coma.

This is usually where we’d list effective treatments or describe how long it takes to recover. But over 97 percent of those infected die. Only four people have survived in the past 50 years in the United States, and scientists are still working to develop an effective treatment. The upside? Only three to four people get infected each year, a rate that’s held steady over time. This is the one item on this list that’s rarer and deadlier than a shark attack.

Some practical tips

These microbes might sound terrifying, but a couple simple, commonsense measures can help keep you and your family safe. Don’t swim at the beach after it rains, as contaminated sewage runs into the oceans and can contain germs. If you have a cut, open wound, or have had diarrhea in the past two weeks, stay out of the water. And the CDC strongly recommends showering before you take a swim. Research shows that the average adult has 0.14 grams of feces attached to their bottom. And children are worse, carrying a whopping 10 grams — the weight of four pennies. Beaches and swimming pools are not wishing wells, and a quick shower beforehand can help ensure that those “pennies” don’t join you.

Once you’re in the water, step out every hour to reapply sunscreen and use the restroom. Be careful not to swallow water — the average adult swallows one tablespoon of water and the average kids swallows 2 1/2. A nose clip can also help keep water from splashing up your nose. When you’re done, dry yourself off, especially around the ears. And, for those who own a pool or hot tub, check chlorine and pH levels twice a day, as per CDC guidelines.

Germs are a fact of life — more so than sharks. But there’s no need to let them stop you from enjoying the water this summer.

  • The comment on shark attack odds ignores exposure time. If you actually spend time swimming in the ocean or are a surfer my guess is that the odds against being attacked are not so great. I have not seen a result that gave the odds for a surfer. Has someone calculated them?

    • Almost surely not — I am not sure there’s an accurate data base of how many hours are spent surfing by how many people.

      Obviously if you never go in the water, your chance of shark bite drops to almost zero (not quite zero, as they have been know to charge the beach, bite fishermen or even in the case of one shark embryo bite a researcher exploring the mother’s abdomen!)

      But exposure time is going to come into play in any of the risks discussed, including the common “less likely than getting hit by lightning” comparison that always accompanies shark-y articles.

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