In the past few years, I have had the privilege of visiting several countries and learning about their health care systems. These have included Australia, Cambodia, Canada, Great Britain, Greece, the Netherlands and Singapore, among others. I have also been asked to participate in several international health care conferences, where I learned about many more systems, from China’s to Italy’s to France’s to Pakistan’s.

One lesson — which is not exactly rocket science — has been that no two health care systems are identical; indeed, variation from hospital to hospital, clinic to clinic, medical practice to medical practice, is a given. Health care is provided in different ways in Minnesota and Boston, for that matter, not to mention Karachi and Capetown. But the most important lesson, to me, is that most health care systems face the same challenges.

The second lesson is that they all address these issues differently; but increasingly, in this age of the Internet, health care leaders around the world are influencing each other and, as a result, their respective systems. For example, China adopted a DRG-based payment system in 2009. Britain’s National Health Service has embraced competition among hospitals. The tireless work of the U.S. Institute for Healthcare Improvement has influenced the quality of care in many countries.

This is not just an international trend. For many years, I have been involved with the National Academy for State Health Policy, which foments the interchange of information among state, municipal and territorial health officials throughout the United States. They share ideas, learn from each other and teach each other.

So the third lesson I would report from my recent ramblings in health care around the world is that many health care systems are beginning to look more and more similar. Sometimes this is a good thing; sometimes, not so much. But it is happening.

So, what are the shared trends?

Population Change

Although the details are specific to each situation, health care systems in many countries are confronted by four demographic issues: aging, youth, diversity and immigration. In much of the developed world, the population is aging quickly. Overall, the median age of human beings on this planet is about 30, but that’s an almost meaningless number. In more developed countries, such as the United States and Japan, as well as Scandinavia and much of Western Europe, the aging of the population and a low birth rate are creating older societies. The median age in most of Western Europe is in the low 40s; in the United States, it’s about 38; in Canada, it’s about 42.

On the other hand, in Afghanistan, Angola and many other countries in Africa and the Middle East, the median age is about 18. In Cambodia, the median age is 24. One reason for these lower numbers is civil conflict and violence, such as in Cambodia, where most of two generations were murdered between 1969 and 1989, and as a result, half the population is younger than age of 25. That makes for a potent workforce now — and a huge headache for its health care system in 40 years. In other countries, especially sub-Saharan Africa, AIDS has wiped out most of a generation. And in many places, marriage and childbearing at a very young age lead to a longer period of fertility.

All of the highest fertility rates are in Africa: The highest, currently, is Niger, at 7.6 children per family; if you don’t know if you are going to live another day, you tend to have a lot of kids. The lowest fertility rates are in developed countries: Bosnia, South Korea, Portugal, Taiwan, Greece, Moldova, Poland, Romania, Singapore and Spain, which are all below 1.5 per woman. India is getting close to 2. The United States is at 1.9. That isn’t even at the replacement rate. Although in rural Cambodia, families are still large, and the birth rate for those in the emerging middle class is at or near replacement level. In China, the world’s most populous country, the government has announced that the one-child-only policy is no longer in effect, which will have a profound effect on its population structure.

But because of growing diversity in so many societies, these numbers do not tell us enough. Non-Latino whites in the United States have a much lower birth rate than other groups. In many rural areas in most countries, families are much larger than in urban settings. It depends on circumstances, culture, religion, the rate of infant mortality, income and a variety of other factors.

And, as has been in the news lately, people are on the move. Folks are trying to get out of areas where civil conflict is ongoing, and are seeking a better life for themselves and their children. And despite the ambivalence of the developed countries about this influx — especially in light of the horrible attacks in Paris — immigrants are likely to be critically necessary, especially in health care, as our populations age and our older citizens want to stay in their homes, but will need many services.

There will be a pile of challenges — generational, cultural and logistical. But all of our health care systems must be aware of the demographics of their societies, of incoming immigrants and of what the health care needs of the future will be.

Chronic Disease

That leads to the obvious second trend, which is the growing presence of chronic disease. This is not confined to older people. Diabetes, for example, is a plague in South and Southeast Asia, and if other countries were more honest in their reporting, we would find that it has become the same in other countries.

It’s not just diabetes. AIDS is now a chronic disease. I suspect, given the air quality and rampant use of tobacco, that chronic obstructive pulmonary disease is an issue in China. Tuberculosis is still a threat in some societies, including the United States. Alzheimer’s and other forms of dementia are becoming more common. And there are many others.

Our health care systems originally were structured to treat acute disease, because when they were formed, few people lived long enough to develop chronic conditions. But that has changed.

Life expectancy has skyrocketed in the last 100 years. Right now, in Japan, Spain, Switzerland, Italy and France, it is older than 80; there are 26 countries in which it is also at or near 80. The United States, at 79, is sneaking up there.

Chronic disease treatment and gerontology have been the stepchildren of most of our health care systems since dirt. We need to get over it. And we need our funding sources to get over it.

Infectious Disease

At the other end of the spectrum is the increase in antibiotic-resistant infectious disease, including recent outbreaks of Ebola fever, Middle East respiratory syndrome and other plagues. Methicillin-resistant Staphylococcus aureus is a constant danger in many hospitals — I have lost two friends to it, one in Wisconsin and one in England. In an article on www.vox.com in 2014, Julia Belluz and Steven Hoffman quoted British researchers who predicted that drug-resistant disease will soon cause the deaths of more people than cancer.

We should have seen this one coming. In 1928, Sir Alexander Fleming accidentally discovered penicillin. He later wrote that we should be very careful in how we use it, because S. aureus and other bugs can mutate and develop resistance. Instead of heeding his words, we did everything from prescribing antibiotics for viral infections (which does no good) to feeding them to cattle. And then we freak out when antibiotic-resistant infections start breaking out. Duh.

This is most definitely a worldwide issue, and one that the health care systems of the world will have to address together. With so much international travel and the emergence of new threats on a regular basis, it is a problem for all of us.

The Cost of Care

Aging societies, more populous societies, and the growing presence of chronic and infectious disease present another challenge: the cost of health care and how we are going to pay for it. When most patients are young and healthy and providers are mostly involved with preventive care and delivering babies, that’s one thing; when most patients are old and have several comorbidities and are likely on government coverage — if they have coverage at all — that’s a different matter.

The true cost of health care in the United States is a mystery, because our accounting systems are directly out of Harry Potter. There are all the complaints about $10 aspirins and so forth, but I think most health care accounting is done with smoke and mirrors. Much of the time, it’s not conscious fraud or anything like it; it’s just that no one knows how to properly track the costs of care, and government requirements make it nearly impossible to even attempt to do so.

My general impression is that the cost of care is just as mysterious worldwide as it is in my country. And everyone blames everyone else. It’s the wrong diagnosis. It’s unnecessary care. It’s fraud. It’s fancy new hospitals. It’s for-profit health care. It’s malpractice litigation. It’s this. It’s that.

Well, we haven’t figured it out yet, but an aging population, the constant influx of technology and the cost of pharmaceuticals might have something to do with it.

Most countries regulate the price of pharmaceuticals. The U.S. policy has long been that the market should decide, which earlier this year led a 32-year-old entrepreneur to purchase rights to a long-used drug and raise its price by more than 5,000 percent. And why shouldn’t he? It’s legal. There have been other such situations. The market does not always decide wisely.

In any case, the pharmaceutical market in the United States is absolutely out of control. But as we move toward genetic-specific and personalized drugs, I don’t think this issue will be confined to the United States.