Two elderly patients are admitted to the hospital. Both have the same condition, maybe pneumonia, maybe a hip fracture. Both are the same age. And both will get more or less the same medical care. The difference is hard to pinpoint, but from the moment they enter the hospital, a doctor can often tell that one will do well and the other poorly.

For a long time, medicine lacked a term to describe the patient who would do poorly, but now we have one: frailty. Frail patients are not sick—they have no particular diagnosable disease. But if they do fall ill or suffer an injury, they are much more likely than non-frail patients to fare badly. In one study that followed a group of elderly patients over seven years, the frail were more than three times likelier to die than their non-frail counterparts.

The precise definition of frailty is still evolving, but most frail people share a number of common traits. Five widely accepted features are weight loss of more than 10 pounds in the last year, frequent exhaustion, low levels of activity, slow gait, and poor grip strength. The syndrome was first defined by Dr. Linda Fried, and colleagues at Johns Hopkins in 2001.

Surgical patients who exhibit just two or three of these characteristics have a much higher risk of poor outcomes. They are twice as likely to suffer complications from surgery, spend an average of 50 percent more time in the hospital, and are three times more likely to be discharged to a skilled-nursing facility instead of to their homes.

The fundamental problem with frailty is a reduced ability to bounce back from biological insults, such as infections and injuries. A relatively minor illness from which a non-frail elderly person would readily recover can throw a frail patient into a downward spiral of additional illnesses, sometimes leading to death.

Consider Mrs. Smith, an independent 85-year-old patient who was still mentally sharp but frail. She developed a urinary-tract infection. A condition that in other patients would have required little more than antibiotics landed her in the hospital, too unstable to walk, intermittently delirious, and dependent on others for care.

In the 19th century, the French physiologist Claude Bernard outlined a concept that came to be called homeostasis, the ability to maintain a functional equilibrium—such as constant body temperature and blood-glucose levels—despite external insults. Robustly healthy people can suffer illness and injuries yet still maintain homeostasis.

By contrast, in the frail, heart, lungs, kidneys, bone marrow, and other biological systems don’t seem able to remain stable in the face of hardship. Even a minor illness or injury can cause one system to fail, and once it does, it often pulls the others down with it.

Frailty represents a huge challenge for the U.S. healthcare system. For one thing, frailty is strongly associated with aging, and the number of older Americans is increasing rapidly. The over-85 population is expected to nearly quadruple by 2050, and among Americans in this age group, about four in 10 are frail.

Our healthcare system is not well-designed to cope with frailty. It is organized around the diagnosis and treatment of particular diseases. But frail people do not necessarily suffer from any single disease. As a result, they often fall through the cracks.

Another challenge is the shortage of physicians who specialize in the care of the elderly. The American Geriatrics Society reports that the U.S. has less than half of the 17,000 geriatricians it needs today. By 2030, the number needed will increase to 30,000.

Frailty is not necessarily a given with old age. There are many robust 90 year olds. Though we don’t fully understand yet what makes one person frail and another person fine, it may be possible to reduce the severity of frailty, or even prevent it entirely, by promoting overall health.

Good nutrition plays an important role. This means eating a well-balanced, varied diet that includes adequate amounts of calories and protein. When elderly friends and relatives are living alone, it can help to check on their eating patterns and arrange to share meals on a regular basis.

Another key factor is exercise. Vigorous physical activity has repeatedly been demonstrated to enhance the function of the brain, the endocrine and immune systems, and skeletal muscle. Even just taking daily walks can be quite effective.

Finally, it is important to promote psychological resilience. Building relationships with others and cultivating a positive, hopeful outlook on life plays a big role in helping many older patients stay active and vital.

To respond effectively to the challenge of frailty, the traditional approach of waiting to intervene until older people develop particular diseases won’t work. But promoting a more robust state of health through living habits could decrease vulnerability. Aging may be inevitable, but frailty is not.