Numerous illnesses for which doctors can find no cause, nor definitely conclude are psychosomatic or imaginary, probably are caused by multiple physical, psychological and social factors interacting in complex ways not yet understood.
There are numerous studies of patients with "Multiple Unexplained Symptom Syndromes". While they are poorly understood, most at least get names: chronic fatigue, irritable bowel syndrome, fibromyalgia, Gulf War Syndrome, Lyme disease, sick building syndrome, multiple chemical sensitivity, sensitivity to the gasoline additive MTBE, or connective tissue disorder, as in women with silicone breast implants. The most common symptoms, at least in patients ill enough to seek medical help, include headaches, fatigue, trouble concentrating or remembering things, nausea, unusual chest pain, shortness of breath, trouble sleeping and musculoskeletal pain. The symptoms often seem worst in patients paying the most attention to them, those living unstimulating, somewhat isolated lives.
Puzzling to researchers is an apparent overlap of symptoms, from one syndrome to another. Another unexplained feature is that while such patients are all exposed to low levels of chemicals, there is rarely see multiple chemical sensitivity in most people.
A new illness has emerged causing fatigue and memory problems among UK sheep farmers, who by law each year must dip sheep in a toxic chemical to kill parasites.
In New Jersey, USA there is an unexplained autism cluster in Brick Township and abnormally high cancer rates among children in Toms River.