News travels quickly on an island, even one the size of Hawaii’s Big Island. So when a twenty-four-year-old construction worker named Graham McCumber ended up in the hospital, in early 2009, it didn’t take long for word to reach Mindi Clark. McCumber, she learned from a friend, had begun experiencing joint stiffness, fatigue, and nausea after eating kale from his garden—the earliest signs, it would turn out, of a mysterious parasitic infection. Within a few days, his symptoms had given way to headaches, double vision, and widespread nerve pain. He fell into a coma. As the illness progressed, McCumber’s left eye turned inward in its socket and his organs started to fail. MRI scans showed that the parasite, a species of nematode called rat lungworm, had burrowed into his brain, leaving tiny tracks through his neural tissue. Doctors warned that he might never recover. Still, as horrible as McCumber’s condition sounded, Clark had other things to focus on, like running her orchid farm and, later, getting treatment for breast cancer. She made an effort to wash her vegetables more thoroughly, but soon she had nearly forgotten about the parasite.

Late last November, Clark came home from a camping trip feeling weak. Headaches followed, then nausea, then a frightening, bone-deep pain. She was sent away from her doctor’s office with a diagnosis of stomach flu. On the second visit—at which point, Clark told me, she felt as though she were being burned alive in short, intense bursts—her physicians tested her for eosinophils, a type of white blood cell that is one of the few reliable indicators of rat-lungworm infection. They prescribed her pain medication and anti-inflammatories and sent her on her way. (Though the nematode can sometimes be eradicated with antiparasitic drugs, the medical community is divided on whether the possible side effects are too risky.) Rat-lungworm infections are highly variable. Depending on the patient and the level of exposure, they can cause loss of limb function, hallucination, and paralysis; Clark experienced terrible pain. Sometimes, it seemed as though her toes were being broken or her A.C.L. torn. “I would just be screaming for help, and my husband couldn’t touch me because it was like I was electrified,” she said.

Until recently, rat lungworm had a reputation as a rare infection whose generally mild symptoms resembled those of the flu. Typically, the parasite makes its primary residence in a rat’s pulmonary arteries, where it lays its eggs. Once the eggs hatch into larvae, the rat excretes them in its feces, at which point a secondary host—usually a snail, a slug, or a flatworm—ingests them. Then another rat eats the secondary host, starting the cycle over again. In some cases, though, the host carries the larvae to a third destination—a head of lettuce destined for a salad, perhaps.

This is where problems crop up. According to Jill Miyamura, of the nonprofit Hawaii Health Information Corporation, incidences of rat-lungworm infection have lately spiked. Her organization recorded one case in 2014, nine in 2015, and twenty-two in 2016. Though the Hawaii State Department of Health cautions that those numbers may be overblown, it has tallied sixteen cases since the start of 2017. Jon Martell, the medical director at Hilo Medical Center, where most of Hawaii’s rat-lungworm sufferers are treated, has seen his caseload double in the past two years. He believes that the situation is more serious than it seems, since the parasite is difficult to detect and is not widely known among doctors. Many moderate infections, he suggested, may be going unreported. “We’re only seeing the worst of the worst,” Martell told me.

Rat lungworm has likely been present in Hawaii for hundreds of years, perhaps brought by rodents that stowed away aboard Polynesian ships. What, then, accounts for the apparent uptick in infections? All signs point to a newcomer to the archipelago—Parmarion martensi, a Southeast Asian semislug, so called because it sports a tiny shell too small to actually live inside. The animal can carry at least twice as many nematode larvae per milligram as other mollusk species; some researchers theorize that its spongy muscle tissue is especially easy for rat lungworm to penetrate. And P. martensi is scrappy. Robbie Hollingsworth, a research entomologist at the U.S. Department of Agriculture, told me that he once visited a woman whose second-floor balcony was infested with semislugs, which had been drawn there by a stash of papaya. “No other slug species I know of would cross the thirty feet to get up there,” he said. When Hollingsworth puts out food for his cat, he added, the semislugs “find it within minutes. How did they do that? It’s kind of like they were waiting in the wings.”

Hawaii has long been a crossroads of international travel and shipping, and its historically spotty import protections have made it especially vulnerable to invasive species such as the semislug. U.S.D.A. records indicate that P. martensi was present in cargo from Malaysia beginning in 2003, though some Big Island residents remember seeing it as early as 1999. Twelve years ago, Hollingsworth and his colleagues performed a study of semislugs on the Big Island and found that three-quarters were infected with rat lungworm, with particularly high numbers in the rural east and on the northern coast. The same team completed a follow-up survey this winter, and though Hollingsworth is waiting for official results from the Centers for Disease Control and Prevention, he has already noted one big change: semislugs are far more widespread than they were a decade ago. So is rat lungworm.

A question that has flummoxed public-health authorities is how the nematode makes its way into humans. The evidence suggests that, in Puna, the rural area where the first rat-lungworm outbreaks were centered, the parasite sometimes reaches people through their drinking water. Semislugs hiding from the Hawaiian sun slip into the rain-catchment tanks that many locals use, drown, and shed their nematodes, which survive long enough to be imbibed by humans. Another possibility is that, as the semislugs crawl over their favorite fruits and vegetables to feed, they leave behind rat-lungworm larvae in their mucus trails. The most likely transmission route, however, is the mollusk itself. Immature semislugs can be smaller than an uncooked grain of rice—easy to miss on a leaf of lettuce or kale. (Clark believes that she was infected during her recovery from breast cancer, when she began consuming large quantities of fresh produce.) Susan Jarvi, a disease specialist at the University of Hawaii at Hilo, told me that adult semislugs sometimes escape detection, too. At one farmers’ market, she said, “a poor woman from Pohoa got a spring roll, and called me frantically, having bit into it and actually eaten half a semislug.”

Jarvi helps run the Rat Lungworm Working Group, a volunteer organization that she founded with Graham McCumber’s mother, Kathleen Howe, in 2011. For years, on a shoestring budget, the group has performed research on the illness and dispensed advice to patients, filling what Howe and Jarvi see as a gap in the government’s response. Funding for their work has died four times in the state legislature.

Recently, though, a series of events has bumped rat lungworm up the public-health priority list. Earlier this spring, P. martensi was confirmed for the first time on Maui. The news coincided with a spike in cases there—six in the past three months, where there had been only two historically—including a pair of honeymooners from California, whose story made headlines. The Department of Health and the Maui Invasive Species Committee quickly organized community meetings throughout the island and formed a working group to address the issue. Keith Kawaoka, the deputy director of the D.O.H., told me that the governor is also planning to expand the scope of a statewide task force originally convened to tackle the Big Island’s rat-lungworm problem. Earlier this year, the Hawaii legislature granted the D.O.H. a million dollars in funding, nominally for work on the parasite but with no specific earmarks. Most of the money, Kawaoka said, will go toward education and outreach: a series of television and radio ads are currently in production. These new initiatives are important, Howe told me, but they can only go so far. “People who know all about rat lungworm and have taken every precaution have still gotten the disease,” she said. “There needs to be research as well to really get the situation under control.”