Anna Phillips is delighted because I’ve just found her favorite parasite, which she had misplaced a year ago.

We are walking through what, at first glance, could be mistaken for an oddly macabre Italian deli. The shelves around us are full of chaotically arrayed jars, which contain what look like formless bits of meat and coiled balls of pasta. But this is actually part of the largest collection of parasites in the country, and on closer inspection, a bundle of tagliatelle is actually a tapeworm. A tangle of capellini is actually a cluster of nematodes. “You can make a lot of food references,” says Phillips. “I try to avoid that, because it ruins food for people.”

But there’s one jar that Phillips is especially keen to find, and after 20 minutes of sweeping the dimly lit shelves with the light of our phones, I spot it. It seems to contain nothing more than a lump of grisly, amorphous tissue, preserved in yellow liquid. But its label, in just a few terse and unconnected words, tells a rich story.

A century ago, the lump was once part of a zebra that was captured in Mombasa, Kenya, and imported to the United States for Barnum and Bailey's Circus. During its stay, it fell sick, and eventually died in quarantine. Upon dissecting its arteries, parasitologist H. W. Graybill found bloodworms—a common and occasionally fatal parasite of horses. Those worms—and a part of the zebra they fed upon—are now in the jar that Phillips is gushing over. She is fascinated by the creatures, but she also loves the way their stories thread through history. She picks up another jar containing a fish tapeworm that was pulled from a dog in New York in 1922. “That was someone’s pet,” Phillips says. “There are so many stories here.”

Phillips is the curator of the National Parasite Collection, a 125-year-old hoard of bloodsucking, fluid-draining, flesh-eating, mind-controlling, and host-castrating creatures that all make a living by living off other animals. These smaller members of this gruesome menagerie are housed in a room full of slides at the Smithsonian National Museum of Natural History. The bigger ones are kept in a separate building on the outskirts of Washington, D.C., which houses much of the Smithsonian’s collection.

The building’s walkways are lined with taxidermied polar bears, leopards, cougars, anacondas, and other superstars of the animal kingdom. But its real treasure is smaller and less charismatic. Put it this way: The largest museum collection in the world belongs to the Smithsonian Institution; the largest Smithsonian collection belongs to the National Museum of Natural History; the largest collection in that museum belongs to the Department of Invertebrate Zoology; and the largest collection in that department is the National Parasite Collection.

The collection was founded in 1892 by two parasitologists from the U.S. Department of Agriculture—Albert Hassall and Charles Wardell Stiles (who waged a successful eradication campaign against hookworms in the American South.) The duo began with whatever they had personally collected, but they were soon flooded with incoming specimens. Doctors would call them up with worms they had yanked out of patients. The local zoo would offer the chance to conduct necropsies on their dead animals.

At first, the collection focused on parasites that infect people and livestock, but it widened its scope after the 1940s. During World War II, U.S. troops found themselves in places with unfamiliar parasitic diseases, and scientists realized that treating those diseases meant understanding the complicated ways in which parasites travel from one host to another. They started looking at parasites in wildlife, and they began appreciating these creatures as important parts of their respective ecosystems—worthy of study in their own right.

Indeed, parasitism is the norm rather than the exception. This lifestyle has evolved independently at least 200 times in the animal kingdom, and around 40 percent of modern animal species are parasites. Most of them are small, but their numbers are astronomical and their influence enormous. In California, a painstaking census of three estuaries revealed that the local parasites weighed as much as all the fish, and outweighed the birds by up to 9 times. Meanwhile, in Japan, researchers estimate that freshwater trout get 60 percent of their energy from suicidal insects, which are compelled to drown themselves by the mind-controlling horsehair worms inside them. And all around the world, parasitic wasps and flies keep pest insects in check, to the delight of farmers and gardeners.

So the National Parasite Collection isn’t a selection of dismissible oddities—it’s really an assembly of the animal kingdom’s unsung heroes and secret overlords. And it’s perhaps one of the largest such assemblies in the world, rivaled only by those in Geneva and London. It’s hard to say exactly how big the collection is because many of its specimens are stuffed into containers by the hundreds. “When you look at a jar of hookworm larvae, it looks like jelly beans in a jar,” says Phillips, going back on her promise to avoid food comparisons. “We say there are around 20 million parasites, but we could easily say 87 million.”

Over the decades, the collection has grown haphazardly, such that samples gathered for a researcher’s dissertation might be sitting pretty next to a tray of rabbit parasites. In 2014, when the Smithsonian took charge of the collection from the USDA, its curators began reorganizing everything. That work will take another year or two. For now, the chaos is frustrating—you can eyeball a row of shelves with no idea what you’re going to find.

The chaos is also delightful for exactly the same reason. People fear parasites, avoid them, kill them. But here, among these shelves, you get to browse them. There’s a tapeworm that was extracted from a python at D.C.’s National Zoo in 1961. There’s a fish tapeworm that was wrenched from the body of an Antarctic seal in 1948. There’s a whole set of tapeworms that were pulled out of reindeer, and Phillips points to the fifth of them. “Dasher, Dancer, Prancer, Vixen, Comet ... that’s Comet’s tapeworm,” she says. “We don’t mention that on the kid tour.”

Some parasites are preserved with the tissues of their hosts, like the bloodworm that fed off the Barnum and Bailey zebra. We see part of a horse stomach with fly larvae attached to it, and flatworms clinging to a piece of cow gut. “It’s important to know how the parasite is associating with the host,” Phillips says. “But also, it would take a hell of a long time to pick each of these out.” Impatience was clearly a problem for yesterday’s parasitologists, who often shoved entire animals into jars. We see a bird, some crabs, a frog in really bad shape, a rat. “There’s something in it, I assume,” says Phillips. Her colleague Kayce Bell reads the label. “Subcutaneous nematode,” she says. “Those are fun.”

Phillips was always interested in animals, and during her college years, she applied for an internship at the American Museum of Natural History in New York. “There were projects on bees, scorpions, and catfish, and I thought, no one’s going to apply for leeches,” she says. That kicked off a career-long fascination with leeches and other parasitic worms that has taken her to six continents, and led to some memorable discoveries. In 2010, she described a new two-inch-long species of leech that had been found inside the noses of three Peruvian children. She called it Tyrannobdella rex—the tyrant leech king, or T. rex for short.

To the study of parasites, Phillips has often given blood, sweat, and tears—sometimes, very literally. For example, she collects leeches “in the usual way”—by finding the right watery habitat, donning shorts, wading in, and making a lot of movement. The leeches come to her. When one bites, she has to lift her leg very slowly because some of them will drop off if they leave the water too quickly. “This is why I do yoga,” she tells me.

Leeches are exceptional, though. In most cases, to collect a parasite, you need to collect its host. Kayce Bell studies the parasites of mammals, and she’s spent a lot of time trapping chipmunks. After euthanizing the animals, she plucks them of fleas, mites, ticks and lice, and then systematically dissects them, scanning every gastrointestinal organ for tapeworms, flukes, and more. Each rodent is more than just an individual: It’s a world, ripe for exploring. “I can do a chipmunk in about 30 minutes,” she says.

This work matters, because parasites are ecologically vital, and poorly studied. And much like the rest of nature, they’re in danger. As hosts die, so do their parasites—and sometimes, conservationists deliberately usher the latter into oblivion. In one notable case, the same people who saved the Californian condor from extinction also wiped out the harmless condor louse, by delousing the last surviving birds.

Humans can also make things harder for parasites in subtler ways. Many of these creatures have free-living stages, where they travel through the world in search of hosts. On these journeys, they’re vulnerable to the same changes in temperature and rainfall that are affecting the entire planet. Global climate change is also forcing animals to move into new areas; that could be a problem for the parasites that depend on those animals, especially if they have complex life cycles that involve several hosts.

Phillips and a team of like-minded scientists recently used the National Parasite Collection to simulate these changes, by looking at how the whereabouts of parasite groups have shifted over time, and how those ranges relate to climate. They estimated that between 5 and 10 percent of parasite species will go extinct by 2070 because of climate change, as will 30 percent of parasitic worms.

These are conservative figures, born of research that has been heavily skewed toward North America, says Carrie Cizauskas from the University of California, Berkeley, who was involved in the study. “There’s hardly any data across Africa. There’s very little parasite research in global-biodiversity hot spots, which are also likely to be parasite-biodiversity hot spots.” With a more detailed portrait of our parasitic planet, Cizauskas says that scientists can work out which groups are most in need of saving.

The very concept of saving parasites is so counterintuitive that Cizauskas and her colleagues found it hard to publish their studies on parasite extinction risk. Reviewers would say, “Why should we conserve these things that everyone thinks are gross?” This attitude seems especially trenchant, but as I’ve written before, history suggests that it can change. Microbes were once seen as germs; now, we’re starting to appreciate how important those in our bodies are for our health. Top predators were once seen as competitors or trophies; now, conservationists talk about protecting or even reintroducing them. Parasites could get the same reputational makeover.

“People ask me if I dreamed about being the curator of the National Parasite Collection when I was five,” Phillips says. “No, not at all. But dreams change over time.”