Medicine has come a very long way in the last century, but it still has a long way to go. Millions of people in the U.S. live with debilitating conditions that, for many reasons, scientists don’t understand. One such condition is systemic exertional intolerance disease (SEID)—an illness better known by its inaccurate common name, chronic fatigue syndrome. SEID is so poorly understood that, to date, there aren’t even any tests to diagnose it. But that may change, as researchers say they’ve found biological markers of the illness in the blood and gut bacteria of people with SEID. Their results were published in the journal Microbiome.

Because so little is known about the physical roots of this condition, and because it predominantly affects women (who are frequently told their unexplained symptoms are imaginary or psychosomatic), many doctors and researchers have argued that it’s actually a mental illness. But a mounting pile of evidence says otherwise.

Even the name of this illness is controversial. The phrase “chronic fatigue syndrome” (CFS) implies that people with the illness are just tired. In fact, the most definitive symptom of this condition is “post-exertional malaise” (another ill-conceived term)—that is, a physiological crash that leaves people completely exhausted after even a small amount of physical or mental activity. In the UK and elsewhere, the condition is called myalgic encephalomyelitis (literally “brain inflammation”), or ME. But brain inflammation doesn’t fully describe the illness either.

Last year, the Institute of Medicine, a federal agency, released a report decrying the lack of research on this disabling condition.

“Remarkably little research funding has been made available to study the etiology, pathophysiology, and effective treatment of this disease, especially given the number of people afflicted,” the report noted. The report’s authors suggested a new name—SEID—but even they admitted that this didn’t quite fit the bill. (For the sake of clarity for this story, we’ll use ME/CFS, an abbreviation that many people with the condition are using until researchers find a more accurate term.)

Gastrointestinal issues like irritable bowel syndrome and heartburn are another common ME/CFS symptom. Previous studies have suggested that something is wrong with the gut bacteria of people with ME/CFS, while others found problems in their immune systems.

To test these ideas, a team of researchers from Cornell University collected blood and stool samples from 48 people with ME/CFS and 39 healthy people. They analyzed the health and diversity of the bacteria in the stool samples and looked for markers of inflammation in their blood.

They found clear differences between the blood and guts of healthy versus sick people. Compared to healthy controls, people with ME/CFS had weaker and less diverse bacterial ecosystems in their guts, as well as higher levels of immune inflammation in their blood. These differences were so clear that the researchers were able to spot nearly 83 percent of the time which participants had ME/CFS just by looking at their bacterial and immune response results.

The researchers believe that these clear biological differences in people with ME/CFS could become a way to diagnose the disease.

Senior author Maureen Hanson is an expert in molecular biology and genetics. “Our work demonstrates that the gut bacterial microbiome in ME/CFS patients isn’t normal, perhaps leading to gastrointestinal and inflammatory symptoms in victims of the disease,” Hanson said in a press statement. “Furthermore, our detection of a biological abnormality provides further evidence against the ridiculous concept that the disease is psychological in origin.”

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