“It’s going to be like breaking up with everyone you know.”

That’s what a colleague predicted three years ago when I took a new job in Chicago and would leave hundreds of my primary care patients in Boston.

She was right. Six months of serial breakups.

I had spent a good part of the prior 15 years developing deep relationships with my patients, learning about their medical problems and, more important, their lives and their hopes. Knowing my patients helped me deliver better medical care and better primary care.

But what is primary care? We often use a definition that includes the “4 C’s.” Primary care is first contact and comprehensive, the place you call first with most of your medical questions and problems. Primary care is coordinated. It is the hub. Your primary care doctor is the quarterback for all of the other care you receive. Finally, primary care is continuous, where you get care over time.

Primary care is good for you. My colleagues and I previously found that Americans with primary care received more high-quality care — recommended cancer screenings, better diabetes care, counseling and more — and had a better experience of care. Others have found that U.S. counties with more primary care doctors have lower mortality. Primary care is associated with fewer emergency department visits, hospitalizations and lower costs. Reliably delivering routine primary care to all Americans would almost certainly improve the nation’s health more than any number of “medical breakthroughs” we often read about.

Now, despite all the benefits of primary care, our new study in JAMA Internal Medicine raises an alarm that primary care in the United States could be slowly dying out.

We found the rates of American adults with a primary care doctor steadily declined between 2002 and 2015. In 2015, a quarter of Americans did not have a primary care physician. Vulnerable groups — blacks, Latinos, the uninsured, the poor and those with less than a high school diploma — were more likely to lack primary care. Americans in the South were less likely than Americans in other regions to have primary care.

Across ages, 20-somethings were the least likely to have primary care, with 44 percent lacking a primary care doctor. While it is true that younger Americans are less likely to need medical care, I am concerned we are setting up the next generation to miss out on recommended, preventive care.

Why the decrease in primary care? Generational difference may play a role. Younger Americans may be more accustomed to internet-enabled, episodic access. The “convenience revolution” has resulted in an explosion of alternative sites of care. Urgent care, retail clinics and online medical companies provide easy options, but these lack the coordination or continuity of primary care. It is easy to make repeated visits, but who is keeping track of you, especially if you have a problem not solved in a visit or two?

Primary care deserves some of the blame. Many primary care practices are not optimized to be accessible or responsive. Primary care is set up, like most health care, because of the way we are paid, for seeing you in person or doing things to you. We largely are not paid for answering phone calls, responding to electronic messages or coordinating care. The sad fact is health systems still make more money by treating sick people than by coordinating care or keeping people healthy.

Beyond the way we pay for primary care, the amount we spend on primary care is part of the problem. In the United States, we spend 5 percent to 7 percent of our health care dollars on primary care, compared with 14 percent in similar countries. Medicare only spends 2 percent to 4 percent on primary care. If we only spend 5 percent of U.S. health care dollars on primary care, is the other 95 percent paying for our failure to spend on primary care?

The supply of primary care physicians could be part of the decrease we found. Demand for primary care should be increasing because of the aging of the population, changes in insurance and other factors, but fewer trainees are going into primary care and doctors are leaving primary care.

My Boston “breakups” were difficult at times, but always amicable.

Starting a new primary care practice in Chicago was hard. I had forgotten how challenging primary care must seem to our medical students and residents when every patient is a new patient. It has taken me three years to start feeling as if I am getting to know a large number of my patients well.

Of course, I hope these deepening relationships will allow me to help my primary care patients stay as healthy as possible. But with fewer of us having primary care — especially the vulnerable and young — I worry we are letting primary care die out and jeopardizing the health of our fellow Americans.