There is a long, grim history of infectious diseases crisscrossing the globe aboard giant ships.

Explorers looking to set up new colonies carried smallpox, measles, and other deadly viruses with them to distant lands. Even the vessels they used to get there contributed to the spread of disease. Infected ballast water from cargo ships traveling to South America, for example, has been blamed for introducing cholera there.

The rise of aviation exacerbated the global spread of disease, effectively shrinking the distance between any one place and the next. (Vaccines have been a major mitigating factor, but there are always emerging diseases, illnesses for which there are no vaccines, and unvaccinated people.) There are tens of thousands of commercial flights across the planet each day, each one carrying a unique stew of germs and vectors. Mosquitoes, which occasionally board and survive international flights, have likely been responsible for bringing malaria to new countries, according to the World Health Organization.

But global shipping still plays a substantial role in the spread of deadly diseases. Consider rat lungworm disease, a grave illness caused by a parasitic worm that invades the human brain. The roundworm that causes the disease, Angiostrongylus cantonensis, is now endemic in the United States. Like many diseases, it is carried by infected rats on container ships, as well as by intermediate hosts like slugs and snails.

Rat lungworm has long been prevalent in parts of Asia and the Caribbean—the first human case of the disease was recorded in Taiwan in 1944—but only recently has it been identified routinely in the United States, including in Hawaii, California, Alabama, Louisiana, Florida, and elsewhere along the Gulf Coast, according to a 2015 study in the Journal of Parasitology. The geographical distribution of this disease has “changed dramatically” in just a few decades, wrote the authors of a separate study, published in the Hawaii Journal of Public Health in 2013.

“So it’s a worm infection introduced into North America through globalization,” said Peter Hotez, the dean of the National School of Tropical Medicine at Baylor College of Medicine.  “Some suggest that it’s due to snails or slugs in the ship ballasts—ships coming from Asia and going through the Panama Canal.”  

Transmission to humans often occurs when people eat intermediate hosts—a tiny,  translucent slug might be imperceptible on a leaf of lettuce that wasn’t adequately washed, for example. Even the slime left behind by an infected slug carries a transmission risk. (Eating raw or undercooked freshwater prawns, crabs, and frogs is also a risk factor.)

In Hawaii, health officials are warning people not to handle backyard slugs or snails with bare hands. Officials there have been closely tracking the disease for years. But a rash of new  cases of the disease has people there rattled. There have been half-a-dozen reported cases of the disease on Maui over a three-month period this spring, three times as many as had been reported in the entire decade up to that point, according to the Maui News.

Rat lungworm disease is notoriously hard to diagnose, largely because there is no blood test that can confirm an infection. (Usually, doctors can determine whether someone is infected based on a patient’s symptoms and exposure history, or a test of cerebrospinal fluid.) There’s no treatment for the disease, though patients are often given painkillers to manage symptoms. Rat lungworm disease can resolve on its own, once the worms die, but in some cases it is fatal.

“I have parasitic meningitis,” said Tricia Mynar, a Maui woman diagnosed with the disease, in an April interview with Honolulu Civil Beat. “The parasites are in the lining of my brain, moving around.”

Mynar described her pain from the disease as worse than childbirth, saying it feels like “somebody opens the top of my head, sets a hot iron inside my brain, then pushes the steam button.”

“Tremors are the hardest part,” she told Honolulu Civil Beat. “They affect me so bad that sometimes I can’t hear my own speech.”

Now, public-health officials elsewhere are anticipating an uptick of cases of the potentially deadly disease across the United States—and trying to figure out just how far it will spread. Global travel, human encroachment into wildlife habitats, and climate change are all factors that will play a role, they say.

“Host specificity of rat lungworms is highly plastic, which contributes to its continuous geographic expansion,” wrote the authors of a 2015 study in the journal Emerging Infectious Diseases. The researchers used a combination of field sampling and predictive modeling to map how carriers and hosts of the disease are likely to spread, and found a “northward range expansion that substantially increases the risk for disease spread within humans and wildlife” in the United States.

One particularly concerning development was the emergence of rat lungworms in Oklahoma, “an area predicted to lack suitable habitat for the parasite,” the authors wrote. A. cantonensis has already been reported in more than 30 countries worldwide, but the presence of the parasite in unexpected regions is, scientists say, a hint at what’s to come.

Scientists are also trying to determine how dramatically climate change and other human-caused changes to the planet, like deforestation, will increase the spread of diseases like rat lungworm. “Most new infections seem to be caused by pathogens already present in the environment, which have been brought out of obscurity, or given selective advantage, by changing ecological or social conditions,” wrote the authors of a 2004 World Health Organization report about the globalization of infectious diseases.

Since 1975, epidemiologists and other scientists have identified more than 30 new human infections. The possibility of new and more damaging strains of existing viruses emerging, like what happened with the Zika virus, pose an additional challenge. What remains to be seen is how at-risk countries approach the relationship between climate change and disease transmission. Those who track the spread of infectious disease find it troubling that many low-income countries with weak public-health infrastructure are already in transmissions zones for a variety of emerging tropical diseases. In other words, the populations that are most at-risk are also least prepared to cope with devastating diseases.

Perhaps more troubling still is that many leaders in rich countries, like the United States, are failing to make a connection between climate change and grave threats to public health—in their own country, and across the globe. The World Health Organization report published more than a decade ago makes this very point, saying that the most consequential health impacts of climate change may have more to do with long-standing challenges within public-health systems rather than as a direct consequence of climate change per se.

On Thursday, U.S. President Donald Trump plans to meet with Chinese President Xi Jinping in Florida. The two leaders are not expected to discuss climate change. Last month, Trump signed an executive order that nullified the previous administration’s efforts to curb climate change, fulfilling one of the central promises of Trump’s presidential campaign.

The two presidents are expected to focus discussions on global trade—however that conversation will almost certainly be focused on economic forces, and not the rats and snails spreading disease across the high seas.