STONE RIDGE, N.Y.—The Hudson Valley of New York, with a breathtaking landscape, bucolic villages and quaint, if struggling, cities, is home to among the nation’s—perhaps the world’s —highest rates of Lyme disease.

The specter of this illness, a quiet plague that has touched thousands here, has changed how valley residents view and experience nature. Weeding the garden, hiking a forest trail, or letting the children run through a field are exercises fraught with danger, in the form of a small, nearly invisible, and all-too common arachnid called the blacklegged tick.

Long spared the scourge of disease-carrying pests that cripple many tropical countries, we in the valley have no such assurance anymore. Nor do those who live in the suburbs, exurbs, even the cities, of many temperate zones of the northern hemisphere. In an era of rising temperatures and diminishing natural diversity, this once-unknown tick is flourishing in more places than ever.

In 1996, Ixodes scapularis, as it is known, had planted a foothold in 396 American counties. By 2015, the tick was established in 842 counties. This does not count another 578 counties—in all nearly half the continental U.S. total—in which the tick has been officially “documented.”

Twenty years ago, Lyme disease was a series of pinpoints on a map, so dark that the Atlantic seaboard from New Jersey to Cape Cod were black. But by 2014, the dots sprouted to form an ominous shadow that obliterates much of Pennsylvania and southern Maine, darkens New York’s Southern Tier to the Great Lakes, jumps borders to Vermont’s Green and New Hampshire’s White Mountains, and liberally peppers Wisconsin, Minnesota and Virginia.

The U.S. Centers for Disease Control and Prevention estimates that 300,000 Americans are infected yearly with Lyme disease, which can cause a rash, joint pain, neurological problems, and, sometimes, a serious heart condition called carditis. A 38-year-old father of three and 17-year-old high school senior, both from Dutchess County, N.Y., died in 2013 after the disease infected their hearts. Nationally, Lyme disease, which lingers in 10 to 20 percent of people treated, costs the healthcare system up to $1.3 billion a year, according to a 2015 study by Johns Hopkins.

Where I live, on the west side of the Hudson River in Ulster County, most everyone knows someone or has had a family member diagnosed with Lyme disease. The young man who plows snow from my driveway wrote at the winter’s start that Lyme disease, acquired as a landscaper, had made it impossible for him to continue working. But, increasingly, we are not alone.

In Europe, disease-ridden castor bean ticks, a relative of those in the U.S., are on the move too, spreading 300 miles north in Sweden and Norway to latitudes that were considered too cold only a generation ago. Prolific and resilient, they are even scaling mountains, climbing 1,300 feet up the Dinaric Alps of Bosnia and Herzegovina and moving to new heights in the Czech Republic and Scotland.

In 1990, the only place the blacklegged tick was found in Canada was in far southern Ontario, on a barrier island across Lake Erie from New York. Less than two decades later, the ticks, hitching rides on the backs of birds and mammals, had moved east, west and north—into Manitoba, southeastern New Brunswick and Nova Scotia. Just as a study in New York found blacklegged ticks moving 125 miles up the Hudson Valley in a 40-year-period, a Canadian study predicts Lyme-toting ticks, will move 150 to 300 miles further north by 2050. Lyme cases in Canada, still officially low, grew nearly six fold from 2009 to 2015.

In that sense, Canada is where New York was decades ago -- facing an illness and a tick against which protections are few and inadequate. The official advice when outdoors: Tuck socks into pants. Wear repellents. Do body checks after coming in. Use special removal tweezers. Watch for a variable red rash but don’t count on getting it—or even seeing it.

Indeed, in the 40 years since the disease emerged in Lyme, Connecticut, science has been powerless to slow these tiny invaders or to protect against them. There is no human vaccine, and control efforts are anemic and few. Canada’s advantage is that, unlike New York in the early days of Lyme disease, it knows what is coming. Hopefully it will not be hit as hard as its southern neighbor.

In a CDC ranking, four of the nation’s 10 counties with the highest Lyme rates, including the top two, were in New York, specifically in the Hudson Valley. Nantucket and Dukes counties, both in Massachusetts, were third and fourth, with Hunterdon County, N.J., fifth. The data, from 2002 to 2006, is likely the last reliable ranking because 19 overwhelmed New York counties have since opted to estimate cases.

Worldwide comparisons of Lyme disease rates are limited by a paucity of consistent data. But what is available suggests that rates in the U.S.—ranging from 219 to 962 cases per 100,000 for the CDC’s top 10 counties—are likely in among the highest globally. In the Netherlands, rates of people diagnosed with the telltale Lyme rash ranged up to 514 per 100,000 in 2014. In areas of Germany and Sweden, studies of patient records found Lyme rates of 261 to 464 per 100,000. In Europe, the highest national rate—315 per 100,000 in 2009 – has been reported in Slovenia, one of few countries to aggressively track cases. Russia, Japan and China are also coping with the disease; residents in a Beijing suburb were reported in a recent study to be “easily infected” and “barely diagnosed and cured.”

Beyond this, there are the other emerging diseases carried by the blacklegged tick, including babesiosis, a parasitic disease akin to malaria, and anaplasmosis, a bacterial infection like Lyme. Cases of these illnesses, sometimes transmitted with or without Lyme disease in a single bite, have increased sharply in New York in the last decade, a harbinger of things to come. Both have at times been fatal.

For many in the Hudson Valley, Lyme disease is quickly and successfully treated with antibiotics. But in many cases, a speck-sized tick wasn’t seen; symptoms were mistaken for something else, and infection spread before treatment began. Decades after its emergence, Lyme disease here is the stuff of shared stories of neighbors debilitated by pain and of children—boys aged 5 to 9 are the most common victim —who have lost precious months of school. It is why we see something else beyond the valley’s beauty.

As spring dawns and the outdoors beckons, this battle of ticks is something to think about. Clinging to a tall grass or a low bush may be the outstretched forelegs of a tick waiting for its next blood meal. If it has already dined on an infected mouse or other mammal key to the chain of infection, that tick, tiny though it is, may give far more than it takes.