A long-standing practice in medicine has been to classify diseases as either non-communicable or communicable. Whereas conditions such as influenza, tuberculosis, and AIDS have long been known to be contagious, disorders and diseases such as colon cancer, acne vulgaris, and type-1 diabetes are generally considered to be non-communicable. Scientific research that has emerged over the most recent decades challenges the validity of this practice and has made it clear that the line between so-called non-communicable and communicable diseases is a lot more blurred than previously thought.
Multicellular organisms such as ourselves are a part of a larger ecological network in which microbial cells and genetic material are transmitted between life forms
By now, it’s well-known, not just among scientists, but also among large parts of the general population, that the human body is home to trillions of tiny organisms, including bacteria and fungi, as well as viruses, which are by some considered to be non-living. We humans are obviously not the only organisms on this planet that house complex microbial ecosystems: many other life forms do as well. The vegetables you ate for dinner last night, the cows you see grazing on the pictures printed on milk cartons, your neighbor’s dog, and the unwelcome insects that are lurking in your basement all co-exist with various forms of microscopic life forms.
These organisms, as well as yourself, are all part of a larger ecosystem of life forms that impact each other in various ways, some of which involve the transmission of microbial cells and genes. Perhaps needless to say, the microbes that reside on the skin of the tiny critters that are lurking in the dark corners of your basement and the bacteria that are hard at work in the guts of grazing cows, where they break down complex polysaccharides such as cellulose, are not going to affect you much, seeing as you are unlikely to get into close contact with them. However, the bacteria that clung to the raw vegetables you ate for dinner last night may have had an impact on the biological system that is you. Some may even have found a spot to live in somewhere in your gut.
And let’s say that you are a good neighbor one night and agree to watch over your neighbor’s dog so as to give its owners a chance to get out and watch the latest and most popular flick at the local cinema without having to worry about Fido. You find the dog to be really cute and end up cuddling with it for many hours. In its desire for closeness, the dog starts drooling all over you, exposing you to some of its bodily fluids (e.g., spit). Just like with the vegetable situation, this may leave an imprint on the ecosystem that is you. The microbes that you may pick up from these types of encounters with other multicellular organisms, whether dead or alive, are obviously not going to dramatically alter the way your body functions. At least most of them aren’t. This is particularly true if you harbor a sturdy, diverse microbiota. However, some of them may subtly change your microbiota, and hence, the way your body functions.
You is a fluid entity/concept. You is malleable, in part because you can pick up microbes from the environment: an environment that includes many different animals and plants, as well as the parasites, bacteria, fungi, viruses, and so forth that they harbor. These latter organisms, which live inside larger organisms, affect many aspects of their hosts’ behavior and biology, including their health and well-being. With these facts in mind, it immediately becomes apparent that health is contagious. It could be argued that many, if not all, conditions that are generally thought of as being non-communicable are actually communicable, at least to some extent.
Obviously, you won’t contract colon cancer, a condition that is closely linked with gut dysbiosis, simply by coming into contact with a person who has this disease. However, if you’re in close proximity with that person (e.g., share a household with him), especially for prolonged periods of time, you may certainly expose yourself to microbial communities that could unfavorably affect your health. This is particularly true if you regularly swap microbes with the diseased person in question via activities such as kissing and sex.
Your susceptibility to being negatively affected by the dysbiotic human microbiotas you are exposed to will largely depend on the resilience and diversity of your own microbiota. If your own microbiota is severely degraded or unstable, you’ll be more susceptible to harm than if it resembles a diverse and flourishing rainforest.
These same principles apply to all diseases that are characterized by microbiota disturbances. Given that disease is pretty much always accompanied by microbiota disruption, it goes without saying that the implications of this type of genetic transmission between organisms are far-ranging. Most humans alive today, in particular those who live under westernized conditions, harbor a microbiota that is suboptimal with respects to what constitutes a truly healthy human microbiota; hence, it’s not really surprising that so many people these days fall sick to heart disease and colon cancer or that virtually all teenagers in industrialized nations get acne.
This begs the question: Have we entered into a vicious cycle in which significant parts of the population are spreading ill-health via transmission of microbes? As crazy as it may seem, the answer to this question is undoubtedly yes. To which extent to this transmission is harming people’s health is elusive; however, there’s no doubt that some people are spreading ill-health via their microbiotas. This is particularly true in the case of mother-infant relationships, which typically involve prolonged close contact between the infant, which is in the process of developing a microbiota, and its mother.
It’s certainly a grim thought that we, via our mass consumption of fast food, widespread use of antibiotics, and modern, unnatural lifestyle habits, may have created a firestorm of illness that’s spreading rapidly across the landscape. What’s important to remember though, is that it’s not just bad health that is, to a certain extent, transmittable between organisms (remember, many other organisms besides humans, as well as non-living components of our world, such as buildings, soil, etc., harbor complex microbial communities, and hence, are a part of the “microbial exchange network”); good health is also, to an extent, transmittable. A healthy partner may actually be the best probiotic there is. In other words, it may be possible to turn the vicious cycle mentioned earlier into a virtuous one. Achieving this on a global scale will obviously be very difficult, seeing as fast food, microbiota-disrupting drugs, and manufactured, “dead” environments have become strongly embedded into our world. However, on a smaller scale, let’s say the scale of your home and family for example, it’s certainly a lot easier to “turn the tide”.
Is there any science to support these claims?
I don’t think any well-informed people will disagree with me when I say that we humans pick up microbes from our environment. This really goes without saying, as we need to get our microbiotas from somewhere. They don’t just magically come into existence. We not only receive quite a bit of bacteria from our parents, in particular the female constituent of this pair; we also pick up microbes from other sources, including the humans we interact with throughout our lives.
For example, a fairly recent study found that approximately 80 million bacteria are transmitted during a 10 second French kiss (1).
The question becomes: How important is this type of transmission of microbes in the context of human health and disease? In my mind, there’s no doubt that it’s very important. I would actually go as far as to say that the swapping of microbes that take place between mothers and their offspring (e.g., during birth and breastfeeding) is in many ways more important in the context of health and disease than the vertical transmission of human genes down the generations.
Emerging science is adding some supportive evidence to this claim. Perhaps needless to say, it’s very difficult to investigate whether a non-communicable disease that’s characterized by some form of dysbiosis, such as acne vulgaris, is contagious. One can’t simply round up a bunch of teens with acne and a bunch without and tell them to make out with each other or live in the same house for prolonged periods of time. I don’t think you’ll get many volunteers for such a project. Also, there are many additional complications with doing a study like this. Among other things, it’s well known that each human carries a unique microbiota; hence, it’s very difficult to control for everything that ideally should be controlled for.
So, I don’t think we should expect to see a study investigating the horizontal transmittability of so-called non-communicable diseases any time soon. This doesn’t mean that the science on this topic is a blank slate though. Research has found that cohabiting family members share microbiota with one another and with their dogs (1, 2). A study that is particularly relevant to this article was published in The World Journal of Gastroenterology just a few months ago. It investigated the influence of cohabitation on the gut microbial communities of partners of ulcerative colitis patients and found that the fecal microbiota communities of the participating ulcerative colitis patients and their spouses were relatively similar. This indicates that the dysbiotic gut microbiotas harbored by the ulcerative colitis patients had been partially transferred from the patients to their spouses (2).
Scientific research has also verified that transmission of microbes involves transmission of health. It has been shown that children who grow up in the presence of farm animals and/or pets tend to be healthier than children who grow up in a less biodiverse environment and that this may largely be attributed to varying levels of microbial exposure (3, 4, 5, 6, 7). Furthermore, it’s well known that the transmission of bacteria from mothers to their offspring affects the health of the offspring.
Many other pieces of evidence also hint that health is transmittable between animals, including humans, via the transfer of invisible organisms. For example, it has been shown that the immune systems of people who live together often converge with respect to their immune cell profiles (8), which is most likely partly due to transmission of microbes. Last but not least, we have evolved ways to detect disease in others and tend to avoid sick and inflamed people and animals (9). This indicates that there is a fitness costs associated with exposure to sick organisms; a cost that is probably partly related to microbial exposure.
I’d argue that we don’t really need hundreds of studies to find out whether health is contagious via transmission of microbes. All we need is a basic understanding of how organisms such as ourselves are put together and function. We already know that we are constantly exposed to microscopic life forms and that some of these life forms become a part of the microbial ecosystems we harbor. We also know that our microbiotas greatly affect our health; that most, if not all, diseases go hand in hand with some type of microbiome disruption; and that dysbiosis is a underlying cause of many health problems. These facts alone should make it clear that it’s time to reconsider the idea that some diseases are wholly non-communicable.