Chronic Fatigue Syndrome — also known as myalgic encephalomyelitis — affects somewhere between 836,000 to 2.5 million people in the United States. However, an overwhelming 90 percent of those people have not been diagnosed.

This is because both patients and doctors are yet to gain a strong understanding and awareness of the disease. Many medical schools do not include CFS as a part of physician training.

The obvious question asks just how we can differentiate it from simply feeling tired. Recognizing the symptoms is quite the challenge, according to Jose G. Montoya of Stanford University Medical Center.

"Not only does the face of the illness vary significantly from one patient to the next, but within the same patient symptoms change over time," Montoya told SELF. He added that neither blood tests nor brain scans can help in confirming the condition. "Patients have no energy, have horrible headaches, back pain, can’t think or find words; all those symptoms are there but the MRI of the brain is normal."

The biggest telltale sign of CFS is a constant, unexplained fatigue which lasts for six months or longer. The affected person may be prone to sore throats, headaches, and concentration problems.

While physical and mental activity may intensify the fatigue — lasting more than 24 hours, according to the Mayo Clinic, it does not seem to improve even after getting rest. 

The severity of the condition varies a lot. Some people experience problems with sleep, thinking, and memory. For others, performing daily tasks like taking a shower or preparing a meal can become exhausting. It should also be noted that CFS is more prevalent in women than men as the former has been associated with a higher risk of autoimmune diseases.

It is worth speaking to a doctor about these symptoms, especially if they are taking a toll on your social life and ability to work. Typically, they will try to figure out if there is something else causing the tiredness such as low iron levels or an underactive thyroid.

While no cure has been established as of yet, treatment with medication may be able to help ease symptoms. For now, more research is needed to understand what the root cause of CFS is.

Recently, scientists from King's College London suggested that it may be tied to an overactive immune system. In their new study of hepatitis C patients, who are given drugs to boost their immune system, 18 of the 55 patients developed chronic fatigue. In other words, a person may have an increased risk of developing CFS after experiencing an infection which has a significant impact on immune system chemicals. This could be hepatitis C, dengue, West Nile, and more.