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The name “mayfly” isn’t exactly accurate, as these inch-long insects aren't technically flies at all. They’re aquatic insects that live on the bottoms of lakes and streams. They usually hatch in May—although that changes based on environmental factors—and fly for just a single day when they emerge in massive swarms of millions of the creatures. While they don’t bite or sting, getting caught in a mayfly swarm is incredibly unpleasant and experts recommend putting Vicks VapoRub under the nose to distract from mayflies’ fishy smell.

Chlorine is often used as a chemical purifier in swimming pools, but it’s also used as a disinfectant in drinking water. This presents a big problem, because chlorine is a poison to higher incidence rates of bladder, breast, and rectal cancers. Even bottled water may contain chlorine, so the easiest way to eliminate the chemical from a personal water supply is to use a carbon-based filter.

There are 120 species of pufferfish in the world and they’re mostly found in tropical and subtropical ocean waters. Pufferfish—aka blowfish—are easy to identify because they puff up into a ball when threatened. Unfortunately, these fish are dangerous, with enough tetrodotoxin in a single pufferfish to kill 30 people. And there’s no known antidote to the poison.

Ranging anywhere from 8 inches to 14 inches long and 10–35 pounds, snapping turtles are quite large and can be found in freshwater mainly within the eastern United States, specifically South Carolina and Georgia. They’re usually docile, but they start to hiss and snap when they feel threatened. It’s best to leave snapping turtles alone in the wild.

Waterborne illness and one of the main bacteria found in water is legionella. It's a bacteria that causes legionellosis which, in turn, becomes Legionnaires' disease and Pontiac fever. The bacteria can be prevented by keeping water between 68 and 122 degrees Fahrenheit. Diligent water system maintenance and disinfection practices can prevent legionella from becoming a problem.

Snakes are scary enough on land, but coming into contact with snakes that can swim brings an added layer of fear to any day at the beach. The yellow-bellied sea snake is venomous and can drift on currents for thousands of miles.

While American crocodiles do exist, their numbers pale in comparison to the saltwater crocodile, found mainly in Australia, India, and Southeast Asia. Weighing 1,000 pounds on average, they lurk just below the surface of the water and snatch anything that moves in their path. Croc attacks are —if a person comes into contact with one of them, they should back away slowly and run in a straight line.

It may look like just another part of the reef or rocks on the bottom of the ocean, but that can do real damage. The fish is covered in little spikes that contain extremely poisonous venom. Found mainly in the waters of the Indian and Pacific Oceans, the venom causes serious pain that can be abated with hot water and administering an antivenom.

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Of all the bacteria to worry about in the water, cryptosporidium is toward the top of the list because it can survive contact with chlorine. Known these parasites are a leading cause of waterborne illness among humans in the United States. Diarrhea, stomach cramps, and dehydration are just a few of the common symptoms one may notice after coming into contact with crypto. Treatment includes antidiarrheal medicine and drinking safe liquids to replace the fluids lost.

Surprisingly, , but the other 5% is what causes problems for jellyfish sting victims. Not all jellyfish are dangerous, but certain species are—including box jellyfish, cannonball jellyfish, lion's mane jellyfish, and other creatively named types. The danger lies in the little sacks called cnidocytes, which distribute poisonous venom through rapid stings. Jellyfish stings can be treated with vinegar and hot water, or a specific venom antidote.

Swimmer’s itch is caused by a microscopic parasite called Schistosomatidae that get released by infected snails. It’s much more frequent in the summer months, and the itch is an allergic reaction that comes from the parasite burrowing into skin, which creates a rash. Corticosteroid creams, oatmeal baths, and baking soda pastes can help treat the skin.

On every coast, there's a danger of rip currents. These currents are fast-moving and can pull a swimmer out to sea at up to 8 feet per second. Human instinct leads people to think swimming back to shore is the way to get to safety, but swimming parallel to shore can get a swimmer out of the rip current’s path and over to safer waters.

They may look pretty, but beware of the flower urchin. While some sea urchins are harmless, the flower urchin (Toxopneustes pileolus) is covered in tentacles that can inject a poisonous venom through soft skin. Found in the Indo-Pacific region of the world’s oceans, the best way to avoid a flower urchin sting is to know how to identify the creature and to stay far away.

While rare in the United States and other advanced nations, cholera still exists and can infect anyone who consumes water tainted with Vibrio cholerae bacteria, especially in areas where poor aquatic sanitation methods are in use. The disease leads to vomiting, diarrhea, and leg cramps, but can be cured through rehydration therapy and antibiotics.

A harmful algal bloom occurs when algae grows out of control. The results can make birds, fish, and mammals sick with the toxins they produce. as they can remove oxygen out of the water, leading to marine life death. More than 400 were reported in the U.S. in 2020, representing a drop from 2019 but a significant overall climb in the decade prior.

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When “sting” is in the name of the creature, there’s a good chance it’s going to be harmful. That’s definitely the case with that can grow to over 6 feet and nearly 800 pounds. A stingray famously of “Crocodile Hunter” Steve Irwin. Lifeguards advise dragging one’s feet on the ocean floor to scatter any stingrays in the area and avoid stepping on one.

Most strains of Escherichia coli are harmless, thriving in the bodies of healthy humans and animals without being noticed, but the 0157:H7 strain is exceptionally harmful and leads to bloody diarrhea, and even death. The bacteria can be spread through fecal matter, so any body of water contaminated with sewage risks having E. coli.

The public’s fear of piranhas dates back to at least Teddy Roosevelt’s days when he wrote about his journey to Brazil and called them “.” Mainly found in freshwater in South America, these carnivorous fish have scalpel-like teeth and can completely strip the skin of an average-sized human in five minutes, assuming 300 to 500 of them could work together on the job.

When it comes to , the word “giant” is a big of a misnomer. They’re just 4 inches long, but with enormous pincers and a hunting style that includes injecting their prey with poison to make them easier to digest, these tiny monsters are terrifying. Found in ponds and creeks around the world, they’re the .

As far as parasites go, . Found all over the globe, this parasite usually enters the body through water contaminated by feces and then hangs out in the digestive track until it's eliminated as poop. Out in the world, giardia is like a horror movie monster that won't die: it can survive for weeks or even months on its own. Be on the lookout for foul-smelling poop, malaise, and abdominal pain as may have entered one's system.

Houseflies don’t bite, but female horseflies most certainly do. Male horseflies aren't able to bite, but drinking blood is necessary for female horseflies' egg production. These insects hatch in late spring or early summer and can usually be found near water. Horseflies are even known to be cannibalistic, feasting on other insects and their larvae.

In tropical waters all over the world, barracudas lie in wait for their next meal. The skinny predator fish can travel 36 miles per hour to devour smaller fish and cut up bigger game with their ultra-sharp teeth. The barracuda has been into the destroyer it is today.

When show up on the nightly news, the culprit is usually norovirus. can be transmitted via water or person-to-person contact, and leads to major intestinal discomfort. The easiest and most effective measure to prevent norovirus is simply washing one’s hands as often as possible.

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are absolutely beautiful to look at, but incredibly toxic to touch. When stepped on or picked up, these sea snails have no problem jamming their teeth into humans to inject their poisonous venom. Mainly found in the Indo-Pacific region, the textile cone snail lives in the sand beneath coral and the area beneath rocks in shallower waters.

Also known as "tangs," surgeonfish have tails that are covered in tiny needle-like spines that can do serious damage to anyone trying to handle the fish. They're all over the Indo-Pacific region, including the Great Barrier Reef. Dory, the character from Pixar's "Finding Nemo" and "Finding Dory," is probably the most famous blue tang in the world.

Adenoviruses can be found in just about any type of water, from rivers to swimming pools to drinking water. Once it enters the human body, cold-like symptoms, pneumonia, or even pink eye can develop. While a vaccine is available to the military, there’s currently available, which means that chlorine is the public’s best hope to fight the disease.

The blue-ringed octopus is objectively adorable and filled with venom that basically paralyzes one’s entire body. Even worse, this vibrant Pacific Ocean creature has enough venom in its tiny body to .

Mosquitoes are certainly annoying pests, leaving itchy bites all over arms and legs, but they're also pollinators and serve as food for a host of other bugs and animals. Mosquitoes live near open bodies of water and one’s is a repellent containing DEET.

Another bacteria transmitted through water, and 450 deaths every year in the United States. The bacteria can infect a water supply and enter a human host to cause diarrhea, fever, and cramps. Amazingly, just one minute of boiling water is enough to kill salmonella before it does any harm.

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The name itself—needlefish—is enough to keep people away. It's called a needlefish because of its slender, narrow shape and long jaw filled with sharp teeth. To make it a touch scarier, to become an even better predator. They can be found in freshwater all over the world and their dangerous pointy shape can be used .