If you’re familiar with anthrax, it’s probably because of what happened in 2001. Letters laced with anthrax spores were sent to news outlets and politicians, killing five people and infecting over a dozen more. But people in southwest Texas were familiar with anthrax long before 2001. It’s all around them.

The Mail Trail Ranch has been in Craig Leonard’s family for over a hundred years.

“Uh, well the rumor is that they came to El Dorado and traded all their cattle in for sheep, and then they drove sheep down there. Yo,” Leonard says.

I caught him while he was driving to San Antonio. The ranch is about 40 miles north of Del Rio. Craig and his dad raise sheep, goats and cattle, and lease the place out to hunters. The area used to be one of the best sheep-raising spots in the country, but the sheep have mostly given way to white-tailed deer for hunting. So the Mail Trail is a lot like most Texas ranches, except in one way: the occasional outbreak of anthrax.

The last big outbreak was around the year 2000, when Leonard was just a kid.

“But it’s very distinct to me because I had just got a new yellow horse, a beautiful gelding,” Leonard says. “And I was going to rope on him, and he’d had one shot of anthrax but he hadn’t had his booster so he contracted anthrax and died. So I remember that pretty vividly, and I don’t think I ever got to ride him.”

The Mail Trail Ranch is located in what’s called the anthrax triangle. The borders are a little rugged, but it starts roughly at the western edge of the Edwards Plateau.

“You know the classic anthrax triangle is from Ozona to Uvalde to Del Rio and back to Ozona,” says Dr. William Edmiston of the Eldorado Animal Clinic. Many of his clients live in the triangle.

“Oh I answer calls probably monthly, I’ll get questions about it especially during the summer and fall. I don’t diagnose many cases because mostly the ranchers that have it know they have it and know what it is and know how to deal with it,” Edmiston says.

Animals get anthrax from spores that live in the soil. The spores really like soil that’s rich in calcium and low in acid – which you find a lot of in southwest Texas. Affected animals have trouble breathing, bleed out of various orifices and then die within 48 hours or so. But there’s a cheap, injectable vaccine, so anthrax rarely affects livestock, and poses almost no threat to humans.

But deer are a different story. It’s hard to give a deer a shot, and there are a lot of deer between Ozona, Uvalde and Del Rio. But here’s what I wanted to know – why does this happen at all? Why does the anthrax triangle exist when other parts of Texas hardly ever see a case of anthrax? Any answers for that, Dr. Edmiston?

“Nope,” he says.

Luckily, other people have had this question too. People who actually have the scientific expertise to answer it. People like Dr. Jason Blackburn of the University of Florida. He’s studied anthrax in animals across the world.

“Montana with bison and elk, Etosha National Park with zebras, wild boars in Ukraine,” Blackburn says.

And in southwest Texas, where Blackburn says anthrax outbreaks are fairly predictable, anthrax spores can live in the soil for years, but usually they’re buried way too deep to be exposed to animals. But that changes with a particular weather pattern.

“So on the Texas landscape we tend to see outbreaks associated with relatively wet springs. So springs with a lot of rains and grasses greening up relatively early in the year,” Blackburn says.

The heavy vegetation growth pushes the anthrax spores a little closer to the surface. Then you need a drought.

Vegetation wilts and animals graze closer to the ground. Then the final step: a big rain to break the drought, and in the process, bring the anthrax spores to ground level.

“So you’ve got the soils softening and you’ve got new vegetation emerging out of the soils, and they could be physically, mechanically moving that bacteria from just below the surface up to the surface and/or even contaminating that vegetation that those herbivores are feeding on,” Blackburn says.

So you’ve got a deer, famished after a drought, who finds a delicious patch of grass, that happens to be contaminated with anthrax. The deer eats the grass, contracts the spores, and then, goodbye, deer. Once the deer decomposes, the bacteria inside of it go right back into the soil, and the cycle continues.

So we know that anthrax likes the soil of southwest Texas. We know that the big rains and droughts we see in Texas help it spread. But how did it get here? It actually came in two waves. And we can trace the first one all the way to Africa. That’s where anthrax was born. But over thousands of years, African mammals gradually migrated north. Some of them did so carrying anthrax.

“To what is modern-day Middle East, South Asia, and the Eurasian continent,” Blackburn says.

The bacteria continued to spread until it got all the way to Spain. When Spanish explorers brought livestock to the New World, they also brought anthrax. First to Mexico, then to Texas, then to the rest of North America. It’s hard to know exactly how.

“What I think happened, and this is pure speculation, is that when it got to the Spanish missions in the 1500s, they didn’t have barbed wire. And so any anthrax they had would have put the local buffalo at risk,” says Dr. Martin Hugh-Jones, an anthrax expert and professor emeritus at Louisiana State University.

Bison and other American animals came in contact with anthrax brought over from Spanish livestock. That accounts for the bacteria’s initial introduction – but not for the anthrax you find in Texas today.

“What we have are modern industrial strains in the western prairie, not an original old strain,” Hugh-Jones says.

By studying the genetics of anthrax in Texas, Hugh-Jones and others determined that the old Spanish strain disappeared here. It was replaced by far younger bacteria.

“What we’re seeing at the moment is the result in the ‘50s and ‘60s of them putting bone meal in the salt mix out for the cattle,” he says.

In the 1950s and 1960s, there were lots of tanneries and mills on the east coast of the United States that processed imported animal hides.

“And some of these had come from animals that had died from anthrax,” Hugh-Jones says.

Here’s what happened: the anthrax would linger on the animal hide. When the hide was washed, the bacteria flowed into a water source. A cow downstream would drink the water contaminated with anthrax, and die. That cow’s bones would be used to make bone meal. “And so you can get a batch of bone meal that hasn’t been properly sterilized, and it’s put out in cattle feed, and bingo,” Hugh-Jones says.

When ranchers spread salt and bone meal to supplement their livestock’s diet, they essentially seeded their land with anthrax, too. Just like anything else you plant, anthrax needs the proper soil to survive. And while it may not have liked the soil in the Panhandle or Central Texas, it felt right at home in the low-acid dirt of the anthrax triangle. Combine that with huge numbers of white-tailed deer to contract and spread the disease, and bingo. A grisly, geographic quirk. But one that doesn’t bother rancher Craig Leonard much.

“It’s just something that if you’re going to live there and you’re going to make a living there, you’re going to have to deal with. But you know, there’s nothing more I want to do in life,” Leonard says.