Evolution has endowed the big-footed snowshoe hare with a particularly nifty skill. Over a period of about 10 weeks, as autumn days shorten in the high peaks and boreal forests, the nimble nocturnal hare transforms itself. Where it was once a tawny brown to match the pine needles and twigs amid which it forages, the hare turns silvery white, just in time for the falling of winter snow. This transformation is no inconsequential feat. Lepus americanus, as it is formally known, is able to jump 10 feet and run at a speed of 27 miles per hour, propelled by powerful hind legs and a fierce instinct to live. But it nonetheless ends up, 86 per cent of the time by one study, as a meal for a lynx, red fox, coyote, or even a goshawk or great horned owl. The change of coat is a way to remain invisible, to hide in the brush or fly over the snow unseen, long enough at least to keep the species going.

Snowshoe hares are widely spread throughout the colder, higher reaches of North America – in the wilderness of western Montana, on the coniferous slopes of Alaska, and in the forbidding reaches of the Canadian Yukon. The Yukon is part of the Beringia, an ancient swathe of territory that linked Siberia and North America by a land bridge that, with the passing of the last Ice Age 11,000 years ago, gave way to the Bering Strait. All manner of mammals, plants and insects ferried east and west across that bridge, creating, over thousands of years, the rich boreal forest. But in this place, north of the 60-degree latitude, the axiom of life coloured by stinging cold, early snow and concrete ribbons of ice has been upended in the cosmic blink of an eye. The average temperature has increased by 2 degrees Celsius in the past half century, and by 4 degrees Celsius in the winter. Glaciers are rapidly receding, releasing ancient torrents of water into Kluane Lake, a 150-square-mile reflecting pool that has been called a crown jewel of the Yukon. Lightning storms, ice jams, forest fires, rain – these things are suddenly more common. Permafrost is disappearing.

Such rapid-fire changes across a broad swathe of northern latitudes are testing the adaptive abilities of the snowshoe hare, however swift and nimble it might be. Snow arrives later. Snow melts earlier. But the hare changes its coat according to a long-set schedule, which is to say that the snowshoe is sometimes snowy white when its element is still robustly brown. And that makes it an easier target for prey. In 2016, wildlife biologists who tracked the hares in a rugged wilderness in Montana gave this phenomenon a name: ‘climate change-induced camouflage mismatch’. The hares moulted as they always had. It’s just that the snow didn’t come. Survival rates dropped by 7 per cent as predation increased.

In order to outwit its newest enemy – warmer winters – snowshoe hares would need something in the order of a natural miracle, what the biologists, writing in the journal Ecology Letters, called an ‘evolutionary rescue’. Like the Yukon, this pristine corner of Montana was projected to lose yet more snow cover; there would be perhaps an additional month of bare forest floor by the middle of this century, on which snowshoe hares would stand out like bright white balloons.

In the tally of species that will evolve or perish as temperatures rise, now consider the moose. The lumbering king of the deer family, known for antlers that can span six feet like giant outstretched fingers, the moose faces a litany of survival threats, from wolves and bears to brain worms and liver fluke parasites. But in the late 1990s in many northern states and Canada, something else began to claim adult cows and bull moose and, in even greater numbers, their single or twin calves.

Lee Kantar is the moose biologist for the state of Maine, which means that he makes a living climbing the rugged terrain of north-central Maine when a GPS collar indicates a moose has died. A lean man with a prominent salt-and-pepper moustache who wears flannel shirts and jeans to work, Kantar tagged 60 moose in January of 2014 around Moosehead Lake in the Maine Highlands. By the end of that year, 12 adults and 22 calves were dead – 57 per cent of the group. When biologists examined the carcasses, they found what they thought was the cause. Calves not even a year old harboured up to 60,000 blood-sucking arthropods known as winter ticks. In Vermont, dead moose were turning up with 100,000 ticks – each. In New Hampshire, the moose population had dropped from 7,500 to 4,500 from the 1990s to 2014, the emaciated bodies of cows, bulls and calves bearing similar infestations of ticks. These magnificent animals were literally being bled to death.

Winter ticks have been known to afflict moose since the late-1800s. In a normal year, a single moose might carry 1,000 or even 20,000 ticks. In a particularly harsh winter, when moose are underfed and weak, anaemia and hypothermia wrought by ticks can make the difference between life and death. Bill Samuel, a biology professor now retired from the University of Alberta, has spent a career studying the moose of North America. He painstakingly counted 149,916 ticks on a moose in Alberta in 1988. In a 2004 book, he recounts episodes of ticks killing moose in Saskatchewan in the spring of 1916, in Nova Scotia and New Brunswick in the 1930s, and in Elk Island National Park in central Alberta at points from the 1940s to the ’90s. Some of the animals were so infested that there was not a tick-free spot in the arachnids’ favoured places – the anus, the inguinal area, the sternum, the withers and lower shoulders. In futile attempts to rid the parasite, these pathetic animals had rubbed against trees to seek relief, losing long, lustrous fur and leaving greyish, mottled patches. They are called ‘ghost moose’.

Moose have long died from disease, predators, hunting and sometimes ticks. But their losses in the early 21st century had a different, more threatening, more consequential implication. In 2015, two environmental organisations, alarmed at population trends, petitioned the United States Secretary of the Interior to have the Midwestern moose listed as an endangered species. In Minnesota, the number of moose dropped by 58 per cent in the decade through to 2015, similar to losses in New England. Environmentalists believe moose could well be eradicated in the Midwest by 2020, with stocks declining precipitously in Wisconsin, Minnesota and Michigan.

Kantar knew that ticks were killing his moose in Maine. What’s becoming clear is why winter ticks had infested his herd, draining half their blood from every available patch of skin. ‘The greatest threat confronting the species,’ declared the non-profits Center for Biological Diversity and Honor the Earth in their 2015 petition to help moose, ‘is climate change.’

Not hunters. Not habitat loss. Not even pollution, though that is important. Moose like and need the cold. They become sluggish when it’s warm, failing to forage as they should, and becoming weak and vulnerable. In the warmer, shorter winters of the US Midwest and Northeast, bumper crops of winter ticks are surviving to wake up when the trees burst to life in earlier springs; they have more time in longer falls to cling in veritable swarms on the edges of high bushes, their legs outstretched, waiting for a ranging, unsuspecting, and wholly unprepared moose. When the moose lie in the snow, they leave carpets of blood from engorged ticks. When a baby moose emerges from the womb in Minnesota, a band of thirsty ticks moves from mother to neonate. The moose shed those fat, flush ticks onto fall and winter ground, and the ticks snuggle into the leaf litter rather than freeze in the snow, as they once might have, reducing tick mortality but upping that of the moose.

Samuel is a careful scientist who does not jump to conclusions, and he sees many forces working together to kill off moose in the finely tuned orchestra that is the outdoors. Wolves, liver fluke, brain worms, unmanaged hunting, habitat loss – they are all part of the picture. Because of how it affects and is affected by those other factors, ‘Climate change,’ he told me, ‘might be the major one.’

‘It’s the ticks.’

Jill Auerbach knows that the winter ticks attached to dead and dying moose pose little threat as a species to humans, whom they aren’t prone to bite. But when news broke of moose losing half their blood to winter ticks, she was horrified and worried. Auerbach, an active woman in her 70s, was bitten in her 40s by a small tick that thrives in the woods, thickets and backyard edges of the county in which she lives, in New York State’s Hudson Valley. She lost 10 years of her life to that tick, had to retire as a highly rated programmer at the nearby IBM plant, and still suffers the aftermath of a case of Lyme disease that was caught too late. ‘It brought me to my knees,’ said Auerbach, among an all-too significant share of people infected with Lyme who suffer long-term symptoms. To her, the rise of winter ticks is one more indicator of an environment out of whack, and so is the more measured, but nonetheless relentless, surge in blacklegged ticks, like the one whose bite haunts her 30 years on.

That other tick, known to scientists as part of the Ixodes genus – in Auerbach’s case, Ixodes scapularis, or blacklegged tick – is spreading across the US and many other countries with startling alacrity. Canada, the United Kingdom, Germany, Scandinavia, Inner Mongolia in China, and the Tula and Moscow regions in Russia: they are all grappling with large and growing numbers of disease-ridden ticks. Infected ticks have been found in urban parks in London, Chicago and Washington, DC, and in the open, green expanses of Killarney National Park in Ireland’s southwest. In western Europe, where case reporting is not standardised, the official case count is 85,000 per year; a 2016 analysis, published in the UK Journal of Public Health in Oxford, put the number at 232,000. Signs of a burgeoning problem are apparent in Japan, Turkey and South Korea, where Lyme cases grew from none in 2010 to 2,000 in 2016. When I asked three Spanish physicians in 2017 where Lyme disease was found in Spain, one said ‘Everywhere’, and the others agreed. One of them, Abel Saldarreaga Marín, had treated forestry workers in Andalucía, where he said symptoms are often managed, perilously, with traditional remedies. In the Netherlands, as elsewhere, warnings to protect Dutch hikers, children and gardeners from bites had failed for years to curb the growing toll, then hit what might simply have been a saturation point, with Ixodes ricinus, or the castor bean tick, inhabiting 54 per cent of Dutch land.

Across the Atlantic Ocean from the Netherlands, the US Centers for Disease Control and Prevention (CDC) in Atlanta issues maps every year showing, by virtue of small black dots, the presence of Lyme disease cases in US counties. The CDC’s 1996 map was the first to officially chart US Lyme cases, although the disease was well along by then. Dots on that inaugural map collectively create an unremitting black smudge along the Atlantic shore from Delaware to Cape Cod. New Jersey, Connecticut, Massachusetts and the lower reaches of New York State – where Auerbach contracted her case – are all inky black. A broken shadow runs along the Wisconsin-Minnesota border, too, with a handful of dots in many heartland states.

But it is the change over the course of 18 years of maps that is telling, depicting the flowering of Lyme in a sort of cartoon flip-book style as it spreads across the Northeast and Midwest of the US. North it goes up New York’s Hudson River Valley and into the state’s Adirondack Mountains, jumping the border to Vermont’s Green and New Hampshire’s White Mountains. West and south it moves great guns into Maryland and northern Virginia. By 2014, the dots consume much of Pennsylvania and darken New York’s Southern Tier to the shores of the Great Lakes and the St Lawrence River. The Upper Midwest is liberally peppered. Dots appear in many other states, too.

In 1996, blacklegged ticks were known to be established – meaning that there were enough counted to breed or they already had – in 396 US counties. By 2015, researchers at CDC reported that the ticks were ensconced in 842 counties, an increase of 113 per cent. Remarkably, the study’s two maps of the continental US – 1996 and 2015 – chart the march of ticks in much the way that the Lyme maps plotted the progress of disease.

Auerbach, who became an advocate with deep knowledge of the ecological issues after her own bout with Lyme disease, has for years ended her emails with: ‘What’s the problem? Well it’s the ticks of course!’ They must be stopped, she believes, and the 2015 CDC map shows why. In it, ticks are seen moving into places that only a decade before had been considered ill-suited to support them, from the Allegheny Mountains to the Mississippi Valley, from western Pennsylvania south and east across Kentucky and Tennessee. In Minnesota and Wisconsin, I scapularis ‘appears to have expanded in all cardinal directions’, the CDC researchers reported in language that was sometimes remarkable and alarming. The ticks have ‘spread inland from the Atlantic seaboard and expanded in both northerly and southerly directions’, they wrote, stopped only to the east by the Atlantic Ocean. Tick movement up the Hudson Valley is ‘recent’ and ‘rapid’, the researchers wrote, their expansion overall: ‘dramatic’. Where there had once been a divide between infestations in the Northeast and Midwest, they concluded, ticks merge ‘to form a single contiguous focus … a shifting landscape of risk for human exposure to medically important ticks’.

Lyme disease emerged in coastal Connecticut in the 1970s, when symptoms akin to rheumatoid arthritis were reported in a circle of children unfortunate enough to be trailblazers of a disease in which early treatment is key to recovery. Diagnoses made late can portend long and difficult sieges of illness – fatigue, joint pain, learning problems, confusion and depression. The parents and guidance counsellors of Lyme children, and the children themselves as young adults, have told me of school years lost to the disease. Children five to nine years old have the highest per-capita Lyme infection rate in the US, while people 60 to 64 years old have the highest hospitalisation rates for it, according to a study of 150 million US insurance records from 2005 to 2010.

The story of the emergence of Lyme disease now, of its rise in dozens of countries around the world and of millions made sick, must be told through the lens of a modern society living in an altered environment. In the last quarter of the 20th century, a delicate array of natural forces indisputably tipped – were tipped, more accurately – to transform Lyme disease from an organism that lingered quietly in the environment for millennia to what it is today: the substance of painful stories shared between mothers; a quandary for doctors who lack good diagnostic tests and clear direction; the object of rancour over studies that discount enduring infection while acknowledging persisting pain.

The CDC does not use the word ‘epidemic’ to describe Lyme disease. It prefers the term ‘endemic’, which it defines as the ‘constant presence and/or usual prevalence of a disease or infectious agent in a population within a geographic area’. But, surely, Lyme was not always present or prevalent. Nor is it confined within well-defined borders. The CDC’s linguistic choice is unfortunate. It serves to minimise the import of a disease that yields some 300,000 to 400,000 new cases in the US each year, is found in at least 30 countries and likely many more, and is growing precipitously around the world. Lyme disease is moving, breaking out, spreading like an epidemic.

The ticks that carry Lyme disease are, like spiders, arachnids not insects. Although they cannot fly or jump, they are, for all practical purposes, climbing mountains, crossing rivers and traversing hundreds, even thousands, of miles to set up housekeeping. These feats are documented by scientists who are ingenious at finding ways to track and count ticks. They drag white flannel sheets across leafy forest carpets, sometimes infusing them with piped-in carbon dioxide, the mammal gas that makes ticks reach up, forelegs outstretched, to snag a passing meal. They catch avian migrants infested with hitchhiking arachnids. They count ticks on the ears of trapped mice and shrews, sometimes getting bitten in the process. They dissect bird nests, reach beneath leaf litter, and scour grassy sand dunes.

When these researchers are lucky, they find data from some other era that proves their hunch that something has changed. In 1956, a scientist named Cvjetanovic in the Bosnian region of the then Socialist Federal Republic of Yugoslavia reported that I ricinus could not survive at altitudes higher than 800 metres above sea level, or about 2,600 feet. But when Jasmin Omeragic of the University of Sarajevo took another look in 2004, collecting 7,085 castor bean ticks in the Dinaric Alps of Bosnia and Herzegovina, he found them living comfortably at 1,190 metres, or 3,900 feet. In 1957 in Šumava, in then-Czechoslovakia, researchers found the ticks could not survive at elevations above 700 metres. By 2001, biologists found them thriving at 1,100 metres. What those early observations pointed to, wrote Jolyon Medlock, a medical entomologist at Public Heath England, and his colleagues in 2013, is ‘clear evidence of an altitudinal expansion of I ricinus’. Put another way, ticks are aggressively moving up. But they are also moving in other ways – and to places more suited than steep slopes to human habitation.

In the Hudson Valley of New York State, a team from the University of Pennsylvania used Ixodes DNA to draw a family tree of blacklegged ticks, much the same way that people use saliva swabs to search for distant ancestors in their genetic code. Studying ticks collected at four locations from 2004 to 2009, the researchers recreated a 125-mile upriver tick migration, similar to that of the colonial Huguenots and Livingstons three centuries earlier. The tree begins in southernmost Yorktown, where the tests showed the ticks residing, give or take, for the previous 57 years. Then, 17 years later, these eight-legged pioneers climb the next rung north, to bucolic Pleasant Valley. Eleven years pass, and they settle in Greenville, in the foothills of the Catskill Mountains, and, 17 years later, emerge in northernmost Guilderland, where Dutch settlers from New Netherland had settled in 1639. While other DNA literally crept in along the way – mate-searching ticks do follow their hearts – by far the most dominant strain at each point along the march was the one from southernmost Yorktown. The DNA data, the researchers wrote, ‘strongly support a progressive south-to-north expansion’. Defying the odds, the ticks had moved to places where it had long been colder and snowier. And they did just fine.

In Europe, ticks are on a similarly relentless march north. In Sweden, researchers studied the range of the castor bean tick from 1994 to 1996 by dragging cloths in 57 locations and querying residents about bites and sightings. They were able to establish a boundary line at about 60°5′N, above which the ticks could not survive. By 2008, the ticks were found to have moved some 300 miles north, mainly along the Baltic coast, to about 66°N. In Norway, the story was repeated.

Twin surveys in 1943 to 1983 found the ticks unable to survive north of 66°N. By 2011, they had travelled 250 miles, to the highest known latitude in Europe, 69°N, Oslo researchers reported, in a record that seems destined to be, if not already, broken. Nicholas Ogden is senior scientist in the National Microbiology Laboratory in the Public Health Agency of Canada. He has watched over the past two decades as blacklegged ticks have leaped the US border in a northerly trek, some 600 miles into Canadian territory. In 1990, the only documented location in Canada where the tick was found was in southern Ontario, in a town called Long Point, which is at the tip of a barrier island jutting into Lake Erie and much closer to New York State than to Ottawa, Toronto or Montreal. Less than two decades later, the ticks had established themselves in a dozen more Canadian locations, including in Manitoba, southeastern New Brunswick and Nova Scotia.

In 2008, Ogden and his colleagues mapped the risk of ticks moving north and predicted ‘possible widespread expansion’ into south-central Canada. By 2015, another study put the forecast farther: Lyme-toting ticks would move about 150 to 300 miles poleward by 2050. That puts Canada in much the same position as the US in the 1980s, and Ogden knows this. The world’s second-largest country, which saw homegrown Lyme cases grow 12-fold from 2009 to 2013, is facing a burgeoning epidemic of Lyme disease. ‘It is becoming a real public-health problem,’ he told me.

In 2015, Ogden and his colleagues employed a novel way to track the destination of ticks on migrating birds. Enter the grey-cheeked thrush, a plain, medium-sized bird and determined skulker that hides in the underbrush, making it prone to collect ticks. Ogden’s team captured the thrush – along with 72 other tick-infested birds – as it crossed the Canadian border on its northward migration. Researchers then studied the molecular composition of its delicate, metal-grey tail feathers. These rectrices, which help to steer the bird in flight, bear a certain fingerprint, an isotope signature from the hydrogen in the water where the bird fledged. Knowing that birds usually return to the place of their birth, scientists concluded that the thrush was destined for the farthest reaches of the study, which covered northern Ontario to the southern Canadian Arctic. Charles Francis, who monitors bird populations for the Canadian Wildlife Service, helped in the study.

‘Very likely there have always been ticks being introduced to northern areas because of migrating birds,’ he said. Only now, more of the ticks they carry are surviving in more places. By 2017, Canadian researchers reported that large swathes of Ontario had converted, as a paper in the journal Remote Sensing put it, from ‘unsustainable to sustainable’ for Lyme-toting ticks. While the snowshoe hares struggle in the rugged Montana wilderness, ticks and their pathogens are thriving in a warming world, colonising more places and multiplying there, just as they did in the last great, post-Ice Age warming. Health officials in Canada 30 years ago told people with Lyme disease that they had almost certainly acquired the infection elsewhere, usually in travel to the US. By the first decade of the 21st century, they had started to hedge their bets.

In 2014, the US Environmental Protection Agency (EPA) issued a 112-page report on the future of the US in a warmer world. It began with a conclusion that had been denied, discounted and politicised in the US for decades, but at last, or perhaps for the moment at least, was accepted as true:

The Earth’s climate is changing. Temperatures are rising, snow and rainfall patterns are shifting, and more extreme climate events – like heavy rainstorms and record high temperatures – are already taking place. Scientists are highly confident that many of these observed changes can be linked to the climbing levels of carbon dioxide and other greenhouse gases in our atmosphere, which are caused by human activities.
The report consists of six sections that attempt to describe and quantify the effects of global climate change ­– on oceans, on the Earth’s glaciers, on forests and lakes, and on people. In the report’s third edition in 2014, the agency included four new ‘indicators’ to track and measure the impact of climate change. These included the number of annual heating- and cooling-degree days (which show that Americans are using more energy to cool rather than to heat); incidence of wildfires; the water level and temperature of the Great Lakes; and last, Lyme disease.

From this point forward, the EPA would track the rate of reported Lyme disease cases across the US as an official outgrowth and barometer of climate change. The tick-borne illness, with perhaps 4 million cases in the US since 1990, is the only disease to be accorded that dubious distinction. In discussing direct health impacts of a warmer Earth, the agency cites two other trends to watch: heat-related deaths, which were estimated at 80,000 in the past three decades, and ragweed pollen seasons that cause painful allergies for millions. But Lyme disease has a singular distinction. It is an illness spread by ticks, the EPA report states, whose ‘populations are influenced by many factors, including climate’.

In states from Maine to Florida and New York to California, across the breadth of southern Canada and in many parts of Europe, once-sweeping woodlands have been reduced and divided, often into idealised forest fragments at the periphery of residential tracts – places where people can be close to, support and observe wildlife. Multitudes live, work and play in or near these green spaces in a new epoch tentatively called the Anthropocene, the era marked by the hand of humanity. The irony is that these adulterated slices of nature and de facto nature preserves are incubators, in many of these places, of Lyme disease. The smaller the patch, in fact, the higher the proportion of diseased ticks, as documented in a study in Dutchess County, New York, where the per-capita rate of Lyme disease is among the world’s highest.

In these fragments, small mammals, such as white-footed mice in North America and garden dormice in Europe, have found havens, thriving in the absence of predators such as foxes. In the language of tick-borne disease, the mouse is quaintly called a ‘host’ for ticks and a ‘reservoir’ of Lyme disease, the place where baby ticks, almost too small to be seen, get their first sip of infection. In city parks, suburban tracts and exurban preserves, people come skin-to-skin with these ticks. In scores of studies, other environmental factors besides climate change, many controlled by human beings, are pointed to as drivers of this epidemic. The slicing and dicing of forests, and the loss of biodiversity that followed, is surely high on a complex and evolving list.

But while there is no single explanation for the 20th-century emergence of Lyme disease, there is ample evidence that climate change has played no small part. At Pinkham Notch, a mountain pass in the northern reaches of the Appalachian Mountains in New Hampshire, snowfall has declined an average of four inches every decade since 1970, and days below freezing have dropped by three per decade since 1960. Lilacs bloom earlier in New Hampshire, a state with vast wilderness areas still intact and a population of about 1 million, and the growing seasons are two to three weeks longer. New Hampshire had the second highest rate of Lyme disease in the US in 2013, after neighbouring Vermont.

In the Krkonoše Mountains in the northern Czech Republic, temperatures increased by 1.4 degrees Celsius in four decades, and I ricinus ticks survive as high as 1,299 metres above sea level. ‘They didn’t decide to go climbing,’ the scientist Michail Kotsyfakis at the University of South Bohemia told me. ‘It’s just that they can survive in these areas.’ In the Montérégie region of southern Québec, extending south from Montréal to the Saint Lawrence River, temperatures have risen since the 1970s by 0.8 degrees Celsius, and the white-footed mouse has thrived in shorter, warmer winters. ‘Its range is rapidly shifting poleward,’ Canadian researchers wrote in 2013, pointing to ‘an increasing body of empirical evidence to support the hypothesis that climate warming is a key driver of Lyme-disease emergence, acting upon many levels of the transmission cycle of the disease.’

The questions are these: did a changing climate cause this epidemic? Or is climate change merely driving this sickness – with the ticks and animals that circulate it – to new places and new peoples? Evidence most certainly supports the latter. The former is trickier. But Lyme disease is distinctive as the first disease to emerge in North America, Europe and China in the age of climate change, the first to become entrenched, widespread and consequential to multitudes of people. It is growing, too, in places such as Australia, where residents are told, as they were in southern Canada and still are in many parts of the US, Canada and Europe, that they must have some other illness besides Lyme disease or, if not, they contracted the infection somewhere else. ‘We’re an island. We have island thinking,’ said Trevor Cheney, a country GP from the mid-north coast of New South Wales, who routinely diagnoses Lyme disease though doctors are told it doesn’t exist in Australia. ‘As if migratory birds’ – which drop ticks far and wide – ‘don’t come there,’ he told me at a conference in Paris.

Such poor advice has cost many Lyme patients valuable time to seek treatment. It grows from a failure, by public health and medical experts, to see the past as the future. Lyme disease is moving to new places, as it has for nearly half a century. In the decades since the children of Lyme, Connecticut, were infected, little progress has been made to control ticks, protect people from bites, test with certainty for the Lyme pathogen Borrelia burgdorferi and, especially, adequately treat the infected. Ixodes ticks – blacklegged, castor bean or otherwise – deserve our respect. They come armed not only with Lyme disease but with a growing menu of microbes: bacterial, viral and parasitic, known and yet unnamed. Ticks can, and sometimes do, deliver two, three or four diseases in one bite. So resourceful are infected ticks that two feeding side by side on the same animal can pass pathogens, one to the other, and never infect the host. So clever is the Lyme pathogen that infected ticks are more efficient at finding prey than uninfected ticks. These ticks might not be able to fly or jump or trek more than a couple of human steps. But they have changed many lives, cost billions in medical care, and coloured a walk in the woods or a child’s romp in the grass, our very relationship with nature, with angst.

This is all the more disturbing when we realise, ultimately, that it is we who unleashed them.