Warning: The subject of this exploration will constrict your blood vessels, choke your windpipe and dispatch you to an early grave, 5 million of you a year. The most lucrative crop the Americas have ever seen, it kept the British at bay, kept the enslaved entrapped, kept Hollywood sexy. Until it didn’t anymore.

Stipulation: Deep bows to the great public health triumph of wrestling Big Tobacco to the mat and changing human behavior. Never before were millions persuaded to give up a highly pleasurable, relatively cheap habit because it was bad for them. And never since.

But: Tobacco itself refuses to die. It’s stubborn. It’s meant to grow here. The seeds are tiny as a flea and germinate like crazy. In less than a month, you can have a robust green crop that’s good for much more than smoking. You can grow vaccines in it. Extract protein from it. Make drugs from it.

Ten years after Maryland became the only state to use its tobacco settlement money to pay hundreds of farmers to quit growing the evil sot-weed, it’s turning out that tobacco has redemptive virtues. Nobody needed to bother exploiting them before; the stuff was so fabulously successful for 400 years as a vice. Even nicotine, the natural and highly addictive chemical in tobacco, has its benefits.

People smoked in part because a cigarette could calm you down and pep you up. Now research studies are exploring exactly how nicotine may safely halt cognitive decline and help those with Alzheimer’s, Parkinson’s, depression, schizophrenia and attention problems. The pure nicotine in the smoking cessation patch used in these studies is extracted from an American product that American farmers know how to grow.

If you drive around Southern Maryland, you can still spy it. Amid the corn mazes and alpaca petting opportunities, the pick-your-own peppers and the thick crop of McMansions, there’ll be a couple acres of plants that look like soldiers — upright, sturdy, tall as a man, with bushy leaves bigger than the blade on a ceiling fan.

You’ll come upon a weathered barn, with some of its boards missing. But on closer examination, you’ll see the slats are opened with a precise symmetry. They let in the air that cures the tobacco hung on its stalks up in the rafters.

Inside the barn, the sheaves, as it has been said, feel like velvet and smell like money.

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The sticks that Sylvester Brady uses to spear up the cut stalks in the field and hang them in the barn — some of them are 100 years old. Brady is 39, and some of the 161 / 2 acres of burley tobacco he tends are off Route 4, right behind the library with its exhibit showing how Calvert County wouldn’t have existed without tobacco, which is why the leaf is so prominent on the county flag. He’s one of a handful of growers who didn’t sign up for the state’s tobacco buyout program. Nearly 900 in the state did, and the first to sign up got their last checks this year.

Like tobacco, Brady is stubborn. He grows the crop under contract with Philip Morris. It’s old-style farm life. There are no computers, no mechanized harvesters. Handle the plant when it’s wet, and it makes you sick as a dog, feverish, cold, shaky. It took researchers some time to determine that was “green tobacco sickness” and to teach workers how to avoid being poisoned by the oil. Among its many attributes, tobacco is a potent natural pesticide.

“You’ve got to love to fool with it to fool with it,” Brady says. “It can try your patience.”

Slaves built the great tobacco fortunes of North Carolina and Virginia and worked the plantations of Southern Maryland. After emancipation, blacks and whites worked as sharecroppers and field laborers. It continued to flourish because of voracious demand. World War I Gen. John J. Pershing pleaded from France in 1918: “You ask me what we need to win this war. I answer tobacco as much as bullets.”

Fifty years later, the lawsuits over the health hazards started.

Gary V. Hodge was finishing up a long stint as executive director of the Tri-County Council for Southern Maryland as then-Gov. Parris N. Glendening was thinking about what to do with Maryland’s share of the billions in tobacco money won through court settlements in the late 1990s.

Tobacco was “part of this deep cultural heritage of one of the dozen oldest counties” in the nation, Hodge says. Its dominance was ending just as bedroom-community development was expanding. He helped come up with an innovative plan to pay Maryland farmers a stipend each year for a decade to tide them over while they looked for other crops. To participate, a farmer had to pledge to quit the crop, unless — and here was the forward-looking loophole — he were to grow tobacco for beneficial purposes. Hodge had some ideas for that.

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Tobacco is often called the white mouse of the plant world, and Nicotiana tabacum has been turned inside out for study. The first studies of nicotine, in which researchers watched how a chicken’s leg twitched when exposed to the drug, were conducted in the late 1800s. They led to the discovery — and naming — of nicotinic receptors in the brain that play a key role in neurotransmitting.

The plant itself has relatively few genes, so it’s easy to manipulate to maximize any trait — low or high nicotine, faster maturity, slower burn rate, tougher leaves for finer cigar-wrapping.

It has a density of high-quality protein, and extracting, purifying and exploiting the value of that protein is where Hodge saw opportunity for Maryland growers. So a team of University of Maryland researchers also have been fooling with tobacco for the past several years.

Agronomist Robert Kratchovil grows a new varietal, hybridized for very low nicotine and very high protein and leaf mass, at a small test plot at the university’s Upper Marlboro farm. This tobacco is harvested several times a growing season, like spinach, and carted off to College Park, where food science professor Martin Lo throws it into a whirring grinding machine that resembles a giant blender. He has experimented with when to harvest for highest protein content and what else might be done with the pulp.

Lo grew up in Taiwan, where, he says, tobacco smoke is used in some non-Western medical applications and where his grandparents always told him to finish his rice. Both inform his quest for new benefits from tobacco. “My belief in science is that we have to be humble,” Lo says. “We only know a little. One development triggers a few new ideas that may lead to something else.”

So far, the project has identified industrial applications for the extracted protein, such as binding agents in paint. The cellulose could be dried and converted into biofuel.

Left over in the plant pulp are dozens of compounds. One of them is solanesol, a precursor of coenzyme Q10, necessary for cell functioning. CoQ10 declines, like nearly everything, with age, sending the middle-aged from the cardiologist to the drugstore. “One bottle at Costco is 32 or 33 dollars!” says Lo, who theorizes that the enzyme could be produced faster and cheaper from tobacco. He brews tea from the pulp, hunts for fermentation possibilities and looks “to use [the tobacco plant] down to the very last drop.”

The next challenge is to build a small bioprocessing facility in the heart of what was tobacco country. Hodge and his partner, Neil Belson, are trying to raise money from investors for that.

Not just in Maryland is bioprocessing seen as the great hope for tobacco farming. North Carolina, the leading state for production, has a venture underway. And other companies are farther along.

Kentucky BioProcessing in Owensboro got a $17.9 million grant from the federal Defense Advanced Research Projects Agency for a project dubbed Blue Angel — to see whether the firm could develop the capacity to fast-track vaccine production in case of pandemic.

Tobacco is under study as a way to fight measles and tooth decay and as a medium for personalized vaccines. “You take a biopsy of a tumor,” explains Kentucky BioProcessing chief executive Hugh Hayden, build a construct to fight it, and tobacco grows so fast that “you go to having a finished [medicine] in a number of weeks.”

Few of these pioneers argue that the virtues of tobacco will replace its cash value as a vice. Reputation reclamation is hard work.

“You can take a crop that once was non-healthy, and still is, and turn it into an alternative that still allows people to make money. It has a lot of promise,” says Ben Beale, the Maryland extension agent in St. Mary’s County, where dozens of Amish and Mennonites still grow tobacco for the cigarette companies.

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Meanwhile, a stubborn plant still rewards its stubborn farmer.

Bobby Anderson is 71, and he refused the buyout. “I didn’t want to sell my heritage, simple as that,” he says. He doesn’t smoke, dip or chew, and neither does his grandson, Lee Pilkerton.

“But tobacco has been very good to me,” Anderson says.

It paid off his house, and it bonded him to Pilkerton, whose daddy died when he was 6. The boy tagged along after his grandfather in the fields and got his own plot at 8, and kept that first money, too.

Pilkerton, who is 30, farms tobacco after he commutes back from his day job, tending the manicured vistas of the federal city as an employee of the National Park Service. He has a handsome house that tobacco helped build, and a wife, Sara, who’s a veterinarian, and a smart 2-year-old named Cora whom he calls the boss, and a set of Belgian horses he takes to compete in tractor pulls.

This is his time of year. Inside the barn, where the fading light glints through those slats and the air is rich with the mellow, roasted smell of drying tobacco, he checks on his crop.

“I love stripping,” he says. “All your hard work, it’s right there.”