Chronic fatigue syndrome (CFS) is characterized by persistent fatigue and other associated symptoms (e.g., musculoskeletal pain, sleep disruption, memory impairments) lasting at least 6 months. The fatigue is not related to other medical conditions, disease processes, or identifiable biologic causes. Sleep, rest, and activity restriction do not improve symptoms. As you can imagine, a syndrome whose main symptom is fatigue can be pretty hard to diagnose, as there are a huge number of other possible issues that can cause it. Since there are so many other conditions that need to be excluded before a diagnosis can be made, it can often take months or years to determine the correct one. Recent studies have revealed that diagnosing CFS may be a bit easier, as it was discovered that virus segments from MLV are found in over 86% of patients.