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Microbes are invisible, so it’s easy to forget that they’re all around us, all the time. They’re in everything we eat and drink, and on every surface that we touch.

A lot of microbes don’t hurt us, but there are some that really can. And even deadly pathogens are closer than you might expect: lurking in tiny rodents, or a tacky decorative fountain. Right here in the United States, there are a surprising amount of diseases hiding in your own backyard.

Here are 6 of them. Chances are you’ve heard of the black death, a plague of such epic proportions that it killed a third of Europe’s population. But what you might not know is that Yersinia pestis, the bacterium that caused the plague, is far from gone.

Adorable furry animals and their fleas carry Y. pestis bacteria, basically everywhere west of Texas in the continental United States. And this bacteria can infect humans and inject our cells with deadly toxins, starting with our immune systems. In bubonic plague, the bacteria multiply in lymph nodes, making them swollen and painful.

This comes with fever, headache, and weakness. Once the infection gets into the bloodstream, it becomes septicemic plague. And if it migrates to the lungs, that’s pneumonic plague.

Now, it’s hard to know how many critters are carrying this ancient terror. But scientists often use coyotes as a sentinel species. Like the classic canary in the coal mine, sentinel species help predict risks to humans, serving as an early warning of potential danger.

Coyotes are scavengers that eat lots of small mammals, both living and dead, which are

common carriers of the plague. So testing them is at least helpful, even if the data’s not perfect. A massive study took blood samples from over 17,000 coyotes from 2005 to 2009, and found that about 10% tested positive for exposure to plague.

Coyotes that had been infected were found in virtually every state west of the 100th meridian, which runs through the middle of North Dakota and Texas. That seems pretty widespread. But luckily, human cases of the plague are pretty rare.

According to the CDC, there have been an average of 7 per year in the U. S. And since 1970, all but one reported case occurred in the West.

Because Y. pestis is a dangerous neighbor, infection control programs target susceptible species like prairie dogs. To protect them, scientists engineered an oral vaccine out of a harmless virus by tacking on Y. pestis antigens — pieces of the bacterium that immune cells use to recognize the disease and fight back. Researchers added the vaccine to treats flavored like sweet potatoes or peanut butter, which were a huge hit with wild prairie dogs in test runs in Colorado and Utah.

So hopefully these cheap and efficient medicinal snacks will help protect more prairie dogs and nearby humans in the future. Small rodents can also carry another ominous disease, called the Sin Nombre virus, or SNV. It was first discovered in 1993 after a cluster of mysterious deaths in relatively young, healthy people in the Four Corners region — where Arizona, New Mexico, Colorado, and Utah meet.

The virus was originally called the Four Corners Virus, but residents objected and it was renamed the Sin Nombre Virus. Which, funnily enough, means “the nameless virus”. Although scientists had known about Hantaviruses in other countries for many years, SNV was the first of several so-called New World Hantaviruses we discovered.

The early symptoms of infection could be mistaken for the flu – fever, chills, and muscle aches. But as the disease progresses, it affects the lungs, so patients drown in their own fluids. It’s not entirely clear what causes this effect.

But researchers think that the virus may send the body’s immune cells into overdrive in the lungs, causing inflammation that damages the tissues. Hantavirus infection is relatively rare – on average, 30 people a year are infected in the U. S.

But it’s quite deadly, killing 35-50% of its victims. We don’t have a vaccine yet, so supportive care is all doctors can do. Cases of SNV have been reported from coast to coast, and it’s mostly carried by the deer mouse, a creature that even the CDC calls “deceptively cute.” Deer mice live pretty much everywhere in North America, although they seem to be especially virus-laden west of the Mississippi river.

For example, a study of nearly 2,000 deer mice in California found that about 12% had antibodies against SNV, indicating that they had been exposed to the disease. But they don’t have obvious symptoms of sickness, and we can get SNV just by breathing in dust contaminated with their infected poop. Fortunately, as far as we know, SNV doesn’t seem to spread from person to person.

So just be a little wary of wild mice, and you should be fine. Plague bacteria and the Sin Nombre virus are often found in remote locations like big national parks where small, furry critters thrive. But nearly half of Americans who got tularemia between 2001 and 2010 lived in urban counties.

And it’s not confined to western states, either: cases have been reported everywhere except Hawaii. Tularemia is caused by a bacterium called Francisella tularensis. This disease is pretty rare, with only 200 to 300 cases a year in the U.

S. But it’s incredibly dangerous. Inhaling as few as 10 microbes is enough to cause a full-blown infection.

And depending on the way the bacteria gets into your body, the symptoms are different. The bacteria trick their way into human cells, then go wild, multiply, and kill them. And they can attack any combination of the skin, lymph glands, eyes, throat, or lungs.

So it’s often misdiagnosed, which is bad news. Tularemia only responds to a few antibiotics, so misdiagnosis can lead to improper

treatment, and more risk of death. Mortality rates are between 2-24%.

F. tularensis bacteria can be transmitted by ticks and deer flies, but Americans have also caught tularemia from pet cats and hamsters. In an outbreak of tularemia in Martha’s Vineyard in 2000, mowing the lawn was even linked to the disease. Probably because they were chopping up dead, infected rodents with the grass.

Even though there is a vaccine for tularemia, it’s only partially effective for now. Researchers are working on it. In the meantime, you can avoid the disease with some common sense: watch out for ticks, and don’t handle wild animals.

And maybe this is a good excuse to put off mowing your lawn. Hypochondriacs beware: you don’t even have to leave your apartment to get Legionnaires’ disease. Cases were reported in every state in 2016.

Legionnaires’ is caused by Legionella bacteria, which usually lives in lakes and streams. But occasionally this bacteria can get into man-made water systems, growing out of control in cooling towers, fountains, and plumbing systems. They thrive even without many nutrients, and can form slimy clusters called biofilms that help them resist disinfectants.

This lets them grow better than other bacteria in man-made systems. From there, the bacteria can become airborne in tiny water droplets, which can spell disaster for unlucky humans that inhale them. Legionella takes hold in lung cells, multiplying and killing them, which causes fevers, difficulty breathing, lung failure, and even death 5-25% of the time.

Legionella is especially problematic in places like hospitals. When people are already sick, their immune systems can’t fight back as well, and there are higher mortality rates. Oddly, Legionnaires’ disease seems to be on the rise in the United States.

Rates of the disease have increased more than 400% since 2000, reaching over 6,000 cases in 2016. But the CDC stresses that this might be because of more awareness and reporting. Despite our best efforts, Legionella keeps popping up in unexpected places, including decorative fountains, grocery store produce misters, hot tubs, and even potting soil.

Thankfully, many cases of Legionnaires’ disease respond to antibiotic treatments, and there have been some efforts to develop a vaccine. So just, like, make sure hot tubs are properly chlorinated before you soak. Cysticercosis is a parasitic infection that’s caused by swallowing tapeworm eggs.

The eggs hatch in your small intestine, and then burrow their way into other organs, where they camp out and form cysts. The severity of this infection can vary a lot. Cysts in muscle tissue may cause no symptoms, or just a slightly sore lump.

But they can also form in the eyes, brain, and spinal cord, which can be debilitating or deadly. Between 2003 and 2012, there were over 18,000 hospitalizations in 42 states for cysts in the brain, called neurocysticercosis. Most were reported in California and Texas.

However, it’s hard to tell how many people got infected in the state where hospitalization happened, or even the United States in general. It can take months or even years after ingesting tapeworm eggs before any symptoms develop. Thankfully, cysticercosis seems to have a relatively low mortality rate.

One study found that from 1990 to 2002, there were only 221 deaths. And, oddly, in many cases the cysts go away on their own. Which is good news, because they’re tricky to treat.

There are drugs that can help destroy cysts, but the inflammatory response from that can be worse than the symptoms. Brain surgery to remove them is also risky. So mostly, washing your hands is a simple and important way to protect yourself.

The final disease on this list is the most rare – there were only 34 cases reported in the U. S. between 2008 and 2017. But it’s also the most dangerous, with a mortality rate of 97%.

The critter to blame is the brain-eating amoeba, also called Naegleria fowleri. This amoeba likes to live in warm freshwater, and infection occurs when swimmers get water up their noses. The amoeba wiggles its way to the brain and releases molecules that destroy brain cells and cause severe swelling.

This usually deadly infection is called primary amoebic meningoencephalitis. Because N. fowleri infection is rare, some people had assumed that the amoeba was fairly rare in the environment. But that doesn’t actually seem to be true.

Originally, we thought it thrived in southern states, with reports of disease from California to Florida. But more recently, there have been cases as far north as Minnesota. Plus, researchers have found the amoeba in recreational lakes in Arizona and reservoirs in Texas… and even treated water systems in Louisiana.

In fact, since 2008, four people have died in the U. S. because of contaminated tap water. The biggest mystery is why hundreds of people can swim in an infected lake and stay totally healthy, while one unlucky person will contract the illness.

So it’s hard to know what to do to stay safe, because treatment options are scarce. There’s only one antimicrobial that may help infected patients. The CDC recommends using boiled or distilled water for nasal rinses, and limiting how much water goes up your nose while swimming in freshwater.

Or, you know, just head to the ocean! N. fowleri can’t survive in salt water. Although these diseases are definitely scary, they’re all pretty rare.

Like, if it’s any comfort, you’re much more likely to die from lightning than from the plague. But knowing what’s out there, we could all be a little more careful. Thanks for watching this episode of SciShow, which is produced by Complexly!

If you want to learn more about the complex things that can affect human health in the. United States, check out our sister channel Healthcare Triage at youtube.com/healthcaretriage. ♩.