I tell a story over on the news side of National Geographic today, about an intrepid team of explorers who plunged into nearly impenetrable Honduran rainforest in order to groundtruth the legend that it concealed a fabled “white city.” They succeeded (and you can read that story in the October issue of the magazine)—but the coda to their tale is that they sacrificed some of their health to do so. Nearly half the team returned home having contracted leishmaniasis, a parasitic tropical disease that causes weeping, disfiguring sores and can be fatal.

As I discovered reporting the story, leishmaniasis is what’s called a “neglected tropical disease,” a loose grouping of illnesses that affect more than 1 billion people worldwide, kill more than half a million people each year—and are almost ignored by drug companies because those victims and the countries they live in are too poor to be a lucrative market for cures. The explorers seeking the lost city discovered this when they grew sick enough to ask the National Institutes of Health for help, and found that the treatments available to them were prolonged, toxic, and didn’t promise a cure.

But here’s the thing: The 20-odd people who were sickened on the Honduran expedition aren’t as rare as we might think. The disease that has afflicted them, which was once limited to the South and Central America, and the Middle East, is gaining a foothold in the United States. Their experience, and their difficulty finding affordable, effective, safe treatment, should serve as warnings that we ought to heed while we can.

Leishmaniasis is unknown to most North Americans. It is transmitted by a parasite and carried by tiny flying insects called sandflies, and there are multiple subspecies of the organism. Depending on which subspecies infects you, you might develop any of three varieties of the illness: cutaneous (involving the skin), mucocutaneous (undermining both the skin, and mucous membranes such as inside the nose and mouth), and visceral (harming the internal organs).

“It’s a disease that tends to occur in conditions of extreme poverty, where there are breakdowns in garbage collection and the sand flies that carry it proliferate, and where people live in poor-quality dwellings,” Dr. Peter Jay Hotez, dean of the National School of Tropical Medicine at Baylor College of Medicine, told me when I was reporting the news story. “It has not been a priority for drug development.”

The places where leishmaniasis is burgeoning are places disrupted by war and civil unrest. Most recently, Hotez said, there have been possibly 100,000 new cases among people who fled the ISIS-occupied zones in Syria or were trafficked out of them as refugees. Those illnesses have been discovered in the Middle East and Europe as refugees arrived. And there’s a separate upsurge among Cubans fleeing that country, who have arrived in the United States overland through Central America and across the US border, and were diagnosed in emergency rooms.

View Images How leishmaniasis is moving north: Cases fell in the yellow circle in earlier years and the red circle after 2000. From Clarke et al., original here.

But the most troubling signal for the future of the disease isn’t either of those outbreaks. Instead, it’s a trickle of cases identified in the American Southwest that indicate the disease is present within our borders, and may be moving north.

In 2012, researchers at the Texas and Oklahoma health departments reported in the American Journal of Tropical Medicine and Hygiene that they had uncovered 13 cases of leishmaniasis over eight years, 2000 to 2007, compared to 29 over almost a century: 1903 to 1996. The people who had the disease had not traveled out of the local area; their cases were what public health calls “autochthonous,” which means “contracted where reported,” or not carried in from somewhere else. The researchers added that several of the 13 had been misdiagnosed at first—so it was likely there were other cases that had not been found.

Why is the disease moving north? The researchers—plus some others who also noticed its advance—say these cases might be due to humans moving into areas where sandflies flourish but urban development was scarce. It’s more likely, though, that its northward advance is linked to climate change: warming temperatures allowing the sandflies to live further north, and drought disrupting the lifecycle of the rodents that the sandflies usually feed on. “As rodent hosts become less available, sand flies may potentially seek blood meals from novel or non-preferred sources, such as humans,” the researchers said.

Only 13 cases, or perhaps a few more: Does this matter? It does. First, because if you’re unfortunate enough to contract it, leishmaniasis is an ugly disease. In Central America, Hotez said, advanced cases are so disfiguring that they make girls unmarriageable, in cultures where marriage is the inescapable social unit, or lead to husbands abandoning their wives. (For images of what advanced leishmaniasis looks like, go here. I’ll spare you.) Second, because treatment is expensive, lengthy, and toxic—several of the explorers had to suspend their treatment because the drugs’ side effects were so bad. A cure isn’t guaranteed.

Ironically, if this disease does move north and infect more North Americans, that might be the thing that prompts drug companies to invest, in better drugs or a vaccine. “We’ve had the genome for years and years, but no one is mining it to look for good targets and embark on drug development,” said Hotez, who is working on a vaccine at Baylor. “It’s the same picture for vaccines. It’s a big market, but not a commercial market.” It looks like a vaccine would be cost-effective, and one vaccine candidate looks promising in very early research.