Our ancestors of yore were plagued by recurrent bouts of malaria, deadly tuberculosis infections, constant syphilis outbreaks and bacteria-laced wounds that never healed. But armed with vaccines and antibiotics, modern-day humans can now avoid or be treated for these and many other communicable diseases — illnesses caused by infectious agents that can be transmitted between people or from animals to people.
Nowadays, most people don't die from communicable diseases but rather those that cannot be passed on to other people. About 41 million people worldwide die each year from cardiovascular disease, cancer, respiratory disease, diabetes or another chronic illness; noncommunicable diseases account for more than 70% of all deaths globally, according to the World Health Organization.
By definition, noncommunicable diseases are thought to arise from a combination of genetic, environmental and lifestyle factors rather than being transmitted by bacteria, fungi or viruses. In recent years, however, scientists have realized that the collection of microbes crawling in and on the human body — known as the microbiome — has a large influence on our health. Could it be that noncommunicable diseases can actually pass between people via the mighty microbiome?
Some scientists think the answer is yes.
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An interesting hypothesis
Communities of microbes make their abode in the human body, and research suggests that these bugs help direct the function of various physiological systems, including metabolism, digestion and immune defense. Scientists don't yet fully understand what distinguishes a healthy microbiome from an unhealthy one, but certain diseases do seem to be linked to a bacterial imbalance in the body.
For instance, people with diabetes, inflammatory bowel disease and cardiovascular disease tend to host a different collection of bacteria in their guts than those without the diseases, according to a report published Jan. 16 in the journal Science. The paper suggests that healthy people could potentially "catch" aspects of these ailments through exposure to these mixed-up microbes.
"It is a radical thought to think that [noncommunicable diseases] might actually be communicable, and [this hypothesis] gives us a whole new way of thinking about these diseases," author B. Brett Finlay, a microbiologist at The University of British Columbia in Vancouver, told Live Science in an email. Several recent studies led Finlay and his colleagues to formulate this hypothesis, but a 2019 study conducted in Fiji really "tipped the scales," he said.
In that study, researchers collected saliva and stool samples from about 290 people living in close proximity to determine the types of bacteria that appeared in their mouths and guts. The results, published in March 2019 in the journal Nature Microbiology, revealed distinct patterns of bacterial transmission within each community, particularly among people living in the same household. While mothers and their children shared many microbes, the microbiomes of spouses seemed to share the most similarities. The team could even predict which study participants were paired up as a couple based on their microbiomes alone.
The Fiji study suggests that at least some elements of the microbiome can be passed between people. But could the transmitted bugs actually drive disease? Quite possibly.
Spouses of people with type 2 diabetes, for example, stand a higher chance of developing the disease themselves within a year of their partner's diagnosis, Finlay noted. In an animal model of the disease, germ-free mice developed diabetic symptoms after receiving a bacteria-laden fecal transplant from a diseased mouse. Similar trends have been uncovered in inflammatory bowel disease, both in human spouses and animal models.
Even cardiovascular disease may be linked to the presence of particular bacteria in the gut, Finlay noted. Certain microbes produce an enzyme that breaks red meat down into a compound called trimethylamine N-oxide (TMAO). People with high concentrations of TMAO in their blood have a high chance of developing cardiovascular disease, and their risk rises if these enzyme-producing bacteria appear in their gut.
Studies show that the bacteria can induce cardiovascular disease if transferred from a human into a mouse, but it's unknown whether the same might occur between people.
Testing the idea
Additional studies hint that more noncommunicable diseases may be influenced by bacteria and that those bacteria may travel between people. "Our lab has shown that early-life microbes impact hugely on asthma ... and we have some very exciting preliminary data with Parkinson’s," Finlay said. Microbes also alter immune function, which may prove relevant to cancer patients whose immune systems fail to recognize and attack tumors in the body, he added.
Obesity, a major risk factor for noncommunicable diseases, also involves potentially transmittable microbes. Lean mice become obese when they receive a fecal transplant from already-obese mice, while humans with obese friends or siblings stand a higher chance of being obese than those who don't have obese friends or siblings. Living in a country with a high obesity rate also raises a person's risk of being obese.
But all of these studies raise a similar question:How can scientists tell which aspects of a disease might be linked to troublesome microbes, as opposed to diet, exercise, genes or environmental factors?
This is a hard question to answer, Finlay said. "Ideally, one does a fecal transfer from a diseased person into a healthy one and causes disease, but of course this can't be done [for ethical reasons]," he said. To test his hypothesis, Finlay and his colleagues will have to rely on animal models and population studies akin to the one conducted in Fiji. If any noncommunicable diseases can be transmitted through microbes, the bugs will meet three criteria: They will appear distinct in diseased people versus healthy people; they will be able to be isolated from a disease host; and they will induce disease when transferred into healthy animals.
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"As we identify mechanisms further, we can actually test these mechanisms, inhibit them ... and really show microbes are involved," Finlay said.
Once scientists clarify how and whether noncommunicable diseases hop between people, they can develop treatments to "correct" diseased microbiomes. Some companies have already begun developing so-called second generation probiotics for inflammatory bowel disease, concocted from a mixture of microbes designed to rebalance the gut microbiome, Finlay said. Dietary changes, pharmaceuticals and, in extreme cases, fecal transplants could also serve as potential treatment options. Fecal transplants involve placing poop from a healthy donor into the colon of another person in order to revitalize their collection of gut bacteria.
"'Repopulating' people with lab-grown mixtures of microbes is probably better [than using fecal transplants], as we know exactly what is going in and don't have to worry about some virus that we haven't discovered yet being transplanted," Finlay said. Fecal transfers will be licensed only for fixing "serious diseases," as the procedure would have to be repeated numerous times, he added.
Scientists still have a lot to learn about how our in-house bacteria shape our health. A slew of fungi and viruses also live in the human body and may offer an additional route for "noncommunicable" diseases to pass from person to person. If Finlay's hypothesis garners support over time, it could lead to an entirely new understanding of noncommunicable disease.
"It has significant public health policy implications," Finlay said, "and further suggests that looking after your own microbes will not only benefit you but also people close to you."
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Originally published on Live Science.