Because ME/CFS is such a poorly understood condition, determining who is at risk for the illness is a difficult task. The Centers for Disease Control has estimated that more than one million Americans have ME/CFS according to the ""Fukuda" diagnostic criteria
developed in 1994, though they estimate that fewer than 20% of those with the illness have been diagnosed. They also estimate that 10 or 20 times as many people may have a condition similar to ME/CFS that doesn't meet the strict requirements for diagnosis. Despite these problems with diagnosis, risk factors have been determined.
ME/CFS often appears to be triggered by a stressor of some sort, but this is not necessary. The apparent stressor is typically physiological, such as a viral infection/flu-like illness or toxic exposure, but may be a mental or physical trauma. The appearance of ME/CFS may follow the suspected trigger closely or be separated by many years.
You are more likely to have ME/CFS if you are an adult female (60% to 85% of diagnosed cases are women), however, men and children can also have it. The incidence of children and adolescents with ME/CFS is difficult to determine because until 2006 there was no pediatric case definition. (See "A Pediatric Case Definition for Myalgic Encephalomyelitis and Chronic Fatigue Syndrome"
Interestingly, ME/CFS has occurred in large outbreaks, such as in a building or neighborhood. This has led a number of scientists to speculate that there may be an infectious cause, such as a virus, or an environmental component, such as toxic exposure, but this has yet to be proven.
In addition, there may be a genetic dimension to the risk of developing ME/CFS. Ongoing genomic research has identified 88 genes whose expression differs significantly in ME/CFS patients by comparison with normal controls, as well as seven likely genomic subtypes characterized by different constellations of symptoms and levels of severity.