Symptoms of diseases don’t always manifest themselves in every single person afflicted by them, and sometimes what looks like a feature of one illness might actually be a sign of another. Diagnoses can be complicated, but as a new study – spotted by ScienceAlert – has pointed out, sometimes we’re simply missing something right in front of our eyes.

Reporting in Human Reproduction, an international team of researchers found that a common symptom of an oft-misunderstood condition, one that affects hundreds of millions of women around the world, is regularly being left to the side by medical professionals: chronic fatigue. It's known to be a symptom, but it rarely appears in medical discussions or large studies.

That condition is endometriosis: It means that the tissue that lines the womb, the endometrium, is found outside of it, in areas like the ovaries or fallopian tubes.

As explained by the UK’s National Health Service (NHS), the symptoms are often pain in your pelvic region – which gets worse during periods – pain when excreting, pain during or after sex, feeling sick, getting irregular bowel movements, having blood in your urine during your period, and difficultly getting pregnant.

Treatment options aren’t ideal. Painkillers can help in the short-term, but hormone medicines and surgery are also available. If left alone, it can be debilitating for many, and even those treated don’t have great odds. One study suggests that 20-50 percent of those that have opted for surgical or medical treatment experience a recurrence of symptoms within five years.

Although it can affect any girl or woman in any part of the world, it mainly appears in those of childbearing age, between 25-35 years of age. As many as 8-10 percent of these women have endometriosis, and yet few have heard of it.

A lack of awareness of the disease, despite its commonality, means that it’s a condition that’s seriously underdiagnosed. Even when it is diagnosed, this often takes an average of 10 years.

Not acknowledging how it manifests itself in its entirety makes dealing with it difficult, which is why this new study is so important. So what did it find?

The team, led by University Hospital Zurich, looked at 1,120 women – 560 with the condition – from a range of ethnic backgrounds who checked in to medical facilities in Austria, Germany, and Switzerland over the past few years. Those with the condition were properly diagnosed; i.e. it was confirmed surgically and by assessing the tissue in detail.

Turns out that “frequent fatigue” was experienced by 50.7 percent of those diagnosed with endometriosis, compared to 22.4 percent in the control group. Although speculative, the team suggest that the endometrial lesions are causing immune system-activating inflammation; this bumps up the body’s use of signaling proteins called cytokines, which have been linked to fatigue symptoms before.

The condition was also associated with a seven-fold increase in insomnia, a four-fold increase in depression, a two-fold increase in pain, and a 1.5-fold increase in occupational stress.

It wasn’t associated with how old the women were, nor how long they'd had the condition for, but clearly, it’s a common symptom that we should not ignore if we hope to better manage and treat endometriosis. The team explains that fatigue could be better put under control if the related symptoms, like depression, are somehow reduced with assistance. If not, the symptoms essentially exacerbate each other.

This isn’t the end of the story. Another study, by the same team, looks at how emotional abuse, neglect, and childhood sexual abuse appear to make endometriosis more likely to occur in adulthood.

As ever, there’s much left to learn, with so many women’s lives depending on it.