By Barbara Ehrenreich, from Natural Causes, which will be published next month by Twelve. Ehrenreich is the author of more than a dozen books, including Nickel and Dimed (Henry Holt). She holds a PhD in cellular immunology.

The pressure to remain fit, slim, and in control of one’s body does not subside with the end of youth — it grows only more insistent as one grows older. Friends, family members, and doctors start nagging the aging person to join a gym, “eat healthy,” or at the very least go for a daily walk. You may have imagined a reclining chair or a hammock awaiting you after decades of stress and physical exertion. But no, your future more likely holds a treadmill and a lat pull, if you can afford access to these devices. You may have retired from paid work, but you have a new job: going to the gym. One of the bossier self-help books for seniors commands:

Exercise six days a week for the rest of your life. Sorry, but that’s it. No negotiations. No give. No excuses. Six days, serious exercise, until you die.

People over the age of fifty-five are now the fastest-growing demographic for gym membership. Mark, a fifty-eight-year-old white-collar worker who goes to my gym, does a six o’clock workout before going to the office, then another after leaving. His goal? “To keep going.” The price of survival is endless toil.

For an exemplar of healthy aging, we are often referred to Jeanne Louise Calment, a Frenchwoman who died in 1997 at the age of 122 — the longest confirmed life span on record. Calment never worked in her life, but it could be said that she worked out. She and her wealthy husband enjoyed tennis, swimming, fencing, hunting, and mountaineering. She took up fencing at the age of 85, and rode a bicycle until her 100th birthday.

Anyone looking for dietary tips will be disappointed; Calment liked beef, fried foods, chocolate, and pound cake. Unthinkable by today’s standards, she smoked cigarettes and sometimes cigars, though anti-smoking advocates should be relieved to know that she suffered from a persistent cough in her final years.

This is “successful aging,” which, except for the huge investment of time it requires, is supposedly indistinguishable from not aging at all. It has many alternative names: “active aging,” “healthy aging,” “productive aging,” “vital aging,” “anti-aging,” and “aging well.” In 2012, the World Health Organization dedicated World Health Day to healthy aging, and the European Union designated that year its Year for Active Aging.

Popular science and self-help books on the topic are proliferating. Among the titles currently available on Amazon are: Successful and Healthy Aging: 101 Best Ways to Feel Younger and Live Longer; Live Long, Die Short: A Guide to Authentic Health and Successful Aging; Do Not Go Gentle: Successful Aging for Baby Boomers and All Generations; Aging Backwards: Reverse the Aging Process and Look 10 Years Younger in 30 Minutes a Day; and, of course, Healthy Aging for Dummies. A major theme is that aging is abnormal and unacceptable. Henry Lodge, a physician and coauthor of Younger Next Year, writes, “The more I looked at the science, the more it became clear that such ailments and deterioration” — heart attacks, strokes, the common cancers, diabetes, most falls, fractures — “are not a normal part of growing old. They are an outrage.”

Who is responsible for this outrage? Well, each of us is individually responsible. All the books in the successful-aging literature insist that a long and healthy life is within the reach of anyone who will submit to the required discipline. It’s up to you and you alone, never mind what scars — from overexertion, genetic defects, or poverty — may be left from your prior existence. Nor is there much concern for the material factors that influence the health of an older person, such as personal wealth or access to transportation and social support.

There is a bright side to aging: declines in ambition, competitiveness, and lust. When Betty Friedan was in her seventies, she wrote a book called The Fountain of Age. As her subjects grew older, she observed, they became “more and more authentically themselves.” They didn’t care anymore what other people thought of them. I can add from my own experience that aging also comes with a refreshing refusal to strive — I feel no need to take on every obligation or opportunity that comes my way.

But even the most ebullient of the elderly eventually come to realize that aging is above all an accumulation of disabilities, often beginning well before Medicare eligibility or the first Social Security check. Vision loss typically begins in one’s forties. Menopause strikes in a woman’s early fifties, along with the hollowing-out of bones. Knee and lower-back pain arise in the forties and fifties, compromising the mobility required for successful aging. The US Census Bureau reports that nearly 40 percent of people aged sixty-five and older suffer from at least one disability, with two thirds of them saying they have difficulty walking or climbing. Yet we soldier on. “You don’t become inactive because you age,” we’ve been told over and over. “You age because you’ve become inactive.”

The goal of successful aging is often described as the “compression of morbidity” into one’s last few years — in other words, a healthy, active life followed by a swift descent into death. But the truly sinister possibility is that for many of us, all the little measures we take to remain fit — all the deprivations and exertions — will lead only to the extension of years spent with crippling and humiliating disabilities. There are no guarantees.

But there are plenty of promises. Skin care products, once content to be “age defying,” are increasingly claiming to be “age reversing,” and we are told by wellness coaches and websites that a youthful appearance is part of feeling good about yourself, which is deemed essential to wellness at any chronological age. Credit for adding beauty — or at least a simulacrum of youthfulness — to the wellness package should go to the new celebrity wellness entrepreneurs, starting with Gwyneth Paltrow, whose company, Goop, has been dispensing beauty and health tips, recipes, and shopping advice since 2008.

The general assumption is that people have plenty of time and money on their hands for, among other things, a $60 skin-rejuvenating pillowcase with patented copper technology or a $5,000 radio-frequency skin-tightening treatment. The wellness entrepreneur du jour, Amanda Bacon, a celebrity by virtue of her Moon Juice products, offers instead of exercise regimens a line of ointments and drinks that are heavy on the exotic and expensive: ho shou wu, silver-needle and calendula tea, pearl, reishi, Cordyceps, Quinton shots, bee pollen, and chaga. The theme here is self-nurturance. The New York Times reporter Molly Young comments:

What Goop (and acolytes like Moon Juice) sell is the notion that it’s not only excusable but worthy for a person to spend hours a day focused on her tiniest mood shifts, food choices, beauty rituals, exercise habits, bathing routines and sleep sched- ule. What they sell is self-absorption as the ultimate luxury product.

In the twentieth century, medical science began to think of aging as a kind of disease rather than a normal stage of the life cycle. Women were used to having their lives medicalized from puberty to menopause, with pregnancy and childbirth as acute episodes requiring intense monitoring and intervention. But since there was no cure for aging, the elderly were pretty much left to their own devices — sometimes taking the form of tonics and elixirs rich in alcohol or cocaine. (These may have been, at least in the short term, highly effective.) Not until the Sixties and Seventies did a researcher come up with a theory of aging at the subcellular level. This was the telomere theory: every time a cell divides, the tips of its chromosomes (telomeres) grow shorter, until eventually further cell division becomes impossible.

The theory had its problems — many types of cells, such as cardiac cells and neurons, reproduce not at all or not very often, yet somehow manage to age. But it also presented a tempting commercial opportunity in the form of drugs that might lengthen and fortify telomeres. Since then, a host of other chemical agents in the aging process have been identified, each with its own proposed nostrum. Free radicals were popular culprits in the Eighties and Nineties, leading to a brief fad for consuming antioxidants such as vitamin E and selenium — to no effect, as it turned out. B vitamins such as folate are thought to encourage cellular health, but their effect on aging is murky at best.

The proposed chemical pathways of aging occur within individual cells, and all are suggestive of decay and entropy. The analogy is often made to the kind of wear and tear that eventually disables the moving parts of a machine — except that cells are not machines, and their moving parts are molecules or clusters of molecules that are subject to perpetual destruction and renewal. Proteins, the fundamental chemical ingredients of cells, are constantly being torn apart and replaced by freshly constructed ones. Some of the key protein players in cellular metabolism have half-lives only minutes long, meaning that there are plenty of opportunities for error, as well as opportunities for correcting those errors. Over time, though, the errors accumulate until the integrity of the cell is compromised. It is then that things get interesting.

Damaged cells send out chemical signals that attract immune cells, which proceed to devour the ailing cells. Some of the immune cells are messy eaters, leaving behind the equivalent of crumbs, which in turn attract more immune cells. Macrophages in particular are drawn to damaged cells; indeed, their chief function in the body, in addition to fighting microbes, is the removal of such compromised cells. Thus the site of cell damage becomes a site of inflammation, where macrophages pile up and attract more macrophages to share in the meal. Inflammation is usually a lifesaving mechanism, but when the target is the body’s own cells or damaged versions thereof, it can lead, gradually, to death.

In 2000, an Italian immunologist named Claudio Franceschi proposed the neologism “inflammaging” to describe the entire organism-wide process of aging. Far from being a simple process of decay originating in individual cells, aging involves the active mobilization of macrophages to deal with proliferating sites of cellular damage. Today, Franceschi’s theory is widely accepted. The hallmark disorders of aging — atherosclerosis, arthritis, Alzheimer’s disease, diabetes, osteoporosis — are all inflammatory diseases, characterized by localized buildup of macrophages. In atherosclerosis, for example, macrophages settle in the arteries that lead to the heart and gorge themselves on lipids until the arteries are blocked. In type 2 diabetes, macrophages accumulate in the pancreas, where they destroy the cells that produce insulin. Osteoporosis involves the activation of bone-dwelling macrophages, called osteocytes, that kill normal bone cells. The inflammation associated with Alzheimer’s was first thought to represent macrophages’ attempts to control the beta-amyloid plaques that clog up the Alzheimer’s brain. But the most recent research suggests that the macrophages actually drive the progression of the disease.

These are not degenerative diseases, not accumulations of errors and cobwebs. They are active and seemingly purposeful attacks by the immune system on the body itself. Why should this happen? Perhaps a better question is: Why shouldn’t it happen? The survival of an older person incapable of reproduction is of no evolutionary consequence. In a Darwinian sense, it might even be better to remove the elderly before they can use up resources that would otherwise go to the young. In that case, you could say that there is something almost altruistic about the diseases of aging. Just as programmed cell death, called apoptosis, cleanly eliminates damaged cells from the body, so do the diseases of aging clear out the clutter of biologically useless older people — only not quite so cleanly. This perspective may be particularly attractive at a time like the present, when the dominant discourse on aging focuses on the deleterious economic effects of aging populations. If we didn’t have inflammatory diseases to get the job done, we might be tempted to turn to euthanasia.

But whatever good deeds immune cells may accomplish in the young, their effect in the elderly is the destruction of the organism. The question of why they do these things might be simplified into a more childish form: Are immune cells good or bad? Friends or foes? For the most part, scientists dodge this question. It would take a lengthy trial to determine the guilt or innocence of the immune system or any cell type within it.

Early in his massive work on the history and philosophy of immunology, The Immune Self, Alfred Tauber states, “The immune self has come to be viewed analogously to a living entity.” What does it mean to say that some part or parts of the body act as a “living entity”? Certainly the cells of the immune system are in constant communication and capable of rather dramatic forms of cooperation. For example, if a macrophage needs to expand its supply of cell-killing digestive enzymes, all it has to do is gobble up a neutrophil and add the neutrophil’s stockpile of enzymes to its own. So the immune system seems to qualify as a system, but does it possess the autonomy we expect to find in a living entity? If so, we should probably call the nervous system a kind of living entity, too, since it is capable of plotting and carrying out the death of the organism on its own, in the form of suicide.

Is the immune system a second, shadow self, assuming the word “self” has not been so degraded by its metaphorical uses as to be meaningless? The best analogy I can come up with would be that it is a symbiont — living in a symbiotic relationship within us, sometimes saving our lives and sometimes destroying us. All we can say for sure is that its agenda does not always concur with ours, and there does not seem to be any command-and-control center within the organism to bring these agendas reliably into harmony. There are many small measures, to be sure — checks and balances, anti- and pro-inflammatory chemical messages — but there is ultimately no one in charge.

We all know how this ends. When the organism dies, as signaled by the cessation of the heartbeat and respiration, not all body cells die simultaneously. The cells’ mitochondria swell, disabled proteins are not replaced, cell membranes start to leak. Macrophages and other phagocytes, which are not wholly dependent on the bloodstream for nutrients, last slightly longer and perhaps enjoy a brief orgy as they rush around devouring damaged cells, but they, too, soon succumb to the lack of oxygen from circulating blood. Bacteria from the gut find their way through leaky membranes to the rest of the body and begin the process of putrefaction.

The muscles, once so carefully sculpted and toned, stiffen when calcium from the dead body leaks into them, causing rigor mortis, then loosen when decomposition sets in. The organs we nurtured with supplements and superfoods abandon their appointed functions. The brain we have tamed with mindfulness exercises liquefies within minutes after the heart stops beating — according to the report of a forensic anthropologist, “It just pours out the ears and bubbles out the mouth.” So much, then, for the hours — and years — you have devoted to fitness.