The threat of Lyme disease grows greater every year, as spring relaxes into summer and parasites the size of poppy seeds crawl in search of blood. Spread primarily by Ixodes scapularis — the black-legged tick, aka the deer tick — Lyme is now the most commonvector-borne illness in the United States. Not a bad showing for a disease that was only identified, from an initial handful of cases, a half century ago. What gives?

A few years ago scientists sought to to gain some insight into Lyme’s provenance by studying Borrelia burgdorferi, the bacterium behind almost all cases of the disease in North America. Over a couple summer months, epidemiologist Katharine S. Walter dragged a square meter of corduroy cloth through the forests of New England for a study she published in October 2017. Her corduroy square collected some 7,000 black-legged ticks and, along with them, the bacteria they carry, whose genetic makeup she and her colleagues set to examining. They were wondering, in essence, where B. burgdorferi had come from — whether its abrupt emergence was the result of a recent genetic mutation, maybe.

It wasn’t: the bacteria, their genetic analysis showed, has been on North America for at least 60,000 years. The arrival of Europeans triggered a chain of events that unlocked its infection potential. Colonists and their descendants, once they got to America, undertook a series of tick-friendly activities: they downed forests and eradicated native predators of tick hosts, and later, filled the atmosphere with so much carbon that the planet warmed. “The ongoing Lyme disease epidemic,” Walter wrote, “was sparked not by evolutionary change but by ecological transformation.”

In the centuries since it was initiated, this transformation has not really slowed down — luckily for the ticks, which have turned out to be some of climate change’s prime beneficiaries. Spring arrives early and winter late; freezes are fewer and softer; temperatures in many places are warmer than usual — or, rather, the new normal is a few degrees above the old. This is all great news for Ixodes et al, and like many of the weird and once unforeseen eventualities of the climate age, it’s a crisis humans all but engineered. Some dystopian visions of the future revolve around humanoid robots or superviruses run amok; ticks and the diseases they spread are less cinematic but, still, a coproduction of humanity and nonhuman nature that is careering out of control. Quiet, malign, slow-moving: ticks are the grotesques of the Anthropocene — emblematic of the way changes in the climate will infiltrate our lives almost without notice at first, attach themselves there, and wreak havoc over time.

Lyme was first identified from a handful of cases in Connecticut in 1977 — B. burgdorferi causes symptoms like fatigue, body aches, and swollen lymph nodes, and attacks joints, muscles, and eventually the brain and heart. Since then, it has spread far beyond its region of origin. Ticks thrive particularly in the simulacra of forest left between developed lots — in short, suburbs and exurbs, where land meets city. Shrunken land has more exposure around its edges; sunlight on the perimeter promotes plant growth that shelters ticks. And the removal of predators like foxes and wolves has encouraged the growth of populations of white-tailed deer, which host and transport ticks, and white-footed mice, which serve as reservoirs for the bacteria — two species that tend to flourish near humans.

Deregulation slowed home-ownership rates for all but the upper classes, who preferred large houses on large lots separated by stands of forest to maintain nominal values of “nature” and “privacy,” a micro-ecosystem uniquely appealing to hungry ticks.

A new paper offers a fascinating case study connecting the spread of Lyme with discrete political and economic developments of the 20th century. In Environmental Sociology, Brent Z. Kaup examines the northern Virginia suburbs surrounding Washington, D.C., where he links Lyme outbreaks to financial deregulation and the Ronald Reagan-era privatization of the defense industry — a weird political-ecological butterfly effect, where choices made in air-conditioned chambers ripple across the landscape. While New Deal reforms and the postwar housing boom enabled an uptick in homeownership among all income brackets, Kaup argues, later deregulation slowed home-ownership rates for all but the upper classes — who preferred, in Virginia as elsewhere, large houses on large lots separated by stands of forest to maintain nominal values of “nature” and “privacy,” a micro-ecosystem uniquely appealing to hungry ticks.

Reagan also privatized defense and intelligence functions, bringing contractors and their employees to the area, further growing the D.C. footprint. “As the acreages of northern Virginia sprawled across the land,” Kaup writes, “DDT use in the area decreased, and forests became more fragmented. The new settlers of northern Virginia thus helped to create the ideal landscapes” for I. scapularis and its mammalian hosts, Kaup wrote. Before the 1980s, nobody had seen a black-legged tick in Virginia. In the 1990s the average annual rate of Lyme diagnoses in the state was 100; from 2000 to 2009 that number had increased to 441, largely in the northern part of the state. More broadly, the CDC found black-legged ticks in 30 percent of U.S. counties in 1998, and half by 2016.

In a report released in May 2018, the CDC said tick-borne diseases doubled between 2004 and 2016, with Lyme constituting 82 percent of the new cases. Impressively, ticks can spread several diseases with one bite. Black-legged ticks cause not just Lyme but infectious diseases like anaplasmosis, ehrlichiosis, babesiosis, and Powassan encephalitis, which the epidemiologist Durland Fish believes might surpass Lyme in terms of its public health implications: Powassan leads to permanent brain damage in half of cases, and fatality in a tenth of infected people. The lone star tick — a southern species whose behavior the CDC calls “very aggressive,” and which has marched up the eastern seaboard all the way to Maine — has begun to infect people with a sugar molecule that causes them to develop an allergy to meat.

The EPA tracks a list of indicators of climate change in the U.S.: growing-season length, wildfires, Great Lakes water levels, and the epidemiological bellwether of a warming globe: Lyme.

Lyme remains the most notorious. Though the CDC is shy to use the term, many including the journalist Mary Beth Pfeiffer describe the disease as epidemic: in fact it’s the title of Pfeiffer’s new book, Lyme: The First Epidemic of Climate Change. As the warming climate provides more comfortable areas for them, I. scapularis and others are heading toward the poles, especially in the northern hemisphere, where it’s estimated ticks’ range increases 28 miles a year. The EPA, Pfeiffer notes, tracks a list of indicators of climate change in the U.S. It includes several factors you’d anticipate — growing-season length, wildfires, Great Lakes water levels — and the epidemiological bellwether of a warming globe: Lyme.

You can drag corduroy across the forest floor to collect ticks or you can simply walk through the woods with your best friend at your heel; this will accomplish, as any dog owner can attest, only slightly less robust results. I spent the last couple of summers in mountainous Tennessee, where ticks were plentiful; they found both the dog and me. Eventually I noticed, at various times of day, phantom crawling sensations on different parts of my body, where I was sure I’d find a tick. Only sometimes was one actually there.

A human version of the vaccine, Lymerix, was approved by the FDA in 1998 and briefly available for sale, before being pulled from the market in 2002.

The difference between me and the dog is that she takes a monthly prophylactic pill against ticks, and even has the option of a canine Lyme vaccine, whereas I don’t. A human version of the vaccine, Lymerix, was approved by the FDA in 1998 and briefly available for sale, before being pulled from the market in 2002. It had a number of problems, including high cost and limited effectiveness, but the drug also ran into the misfortune of debuting amid a swell of anti-vaccine sentiment in the U.S. following the publication of Andrew Wakefield’s (since retracted) Lancet article falsely linking vaccination with autism. Subsequent anecdotal reports connected Lymerix to the onset of symptoms of arthritis; these were later debunked, but the drug’s manufacturer had by then taken it off the market.

Just as humans have done a terrific job of engineering the ideal conditions for ticks and tick-borne diseases, the situation allows for skillful exploitation of modern-day anxieties and controversies. Just as debunked studies about negative effects of vaccinations gave rise to people reporting those effects, the Lyme epidemic has given rise to a contingent of people who claim to suffer from “chronic Lyme disease,” which describes a set of physical symptoms that have not actually been linked to a positive test for Lyme-related bacterial infection, which is required for a Lyme diagnosis of any kind. The CDC disavows the concept of “chronic Lyme”; for the small population of patients who have tested positive for Lyme-related bacteria, been treated, and no longer test positive for infection but still have lingering symptoms such as fatigue and brain fog, the CDC prefers the term “post-treatment Lyme disease syndrome.” (Adding to the confusion, sometimes the terms are used interchangeably, though the distinction between the two cases based on lab tests is settled, uncontroversial science among doctors.)

When it comes to diagnoses like “chronic Lyme” that have “no plausible laboratory, clinical, or epidemiological evidence,” Rachel Pearson, MD, Ph. D suggested that physicians could better practice “epistemological gentleness” in fielding patient concerns by understanding “chronic Lyme” as a disease not related to Borrelia — that is, not bacterial, not tick-borne — but “real” in terms of the symptoms that patients describe, and responsive to “intensive treatment with placebos” in blind tests (a different name for the condition might help, too). In any event, Lyme remains shockingly underdiagnosed: around 30,000 cases reported annually, a tenth of what the CDC estimates is the actual rate.

Tick-borne diseases also prey upon our uneasy relationship with nature, which is its own worrying long-term consequence: Ticks alienate us from the natural world when we can least afford to be, just as we’re starting to fully appreciate the horror of what we’re doing to it. The forest stores both carbon and magic, and ticks give reason for pause at its borders — suggesting there might be someplace a bit less hazardous to take a walk, or that we might just go on destroying the outdoor unknown wherever it’s not serving our immediate goals. What’s the alternative, beside bug spray?

Already I can see a change in my own behavior — slight, but in ways I fear will add up over time. The prospect of a walk in the woods has become cumbersome, fraught, involving not just the act itself but the lonely ritual afterward in front of the bathroom mirror, peering more intently than I’d rather at various body parts, looking for what seems amiss. And then afterwards, the worry I’ve missed something; and the calculation, next time around, of how much of this is really worth it. Alone, this isn’t a great tragedy, but it portends something deeply sad: the world warms by mere degrees, the oceans rise a millimeter at a time, and our steps through the forest grow shorter and shorter, until they stop altogether.