CFS is caused by the food additive, carrageenan

Carrageenan is a particular, low-growing and clumping type of seaweed first described as being from the SW corner of Ireland, although nowadays the same or similar compound is derived from various seaweeds (which are actually more dangerous than the original).
Iceland, where the first modern researched outbreak of chronic fatigue syndrome - called Akureyri or Iceland disease - occurred during the immediate WWII post-war years had a long history of Icelanders eating seaweed when times were hard. As indeed all the far northern islands in this area also did. There is also a history of famines, starvation, of sickness, of sailors or other islanders washed up on the shores of various islands having no other option but to eat seaweed. It takes about seven years of eating seaweed before the acute phase of CFS to establish itself. Icelanders also used to regularly make a milk pudding just like a blancmange from a seaweed called dulse (dulse pudding) until recent times when Iceland got a lot richer. During the four to five WWII years, Akureyri as the furthest northern town in Iceland was almost completely cut off from the richer capital, Reykavik, and the American/British base which brought supplies in by plane and ship. The only road was mainly unmetalled; the distance between the two towns was about the same as the distance between London and Glasgow; fishing was severely curtailed as the Germans used this far northern route to get into the Atlantic from the northern German-controlled ports in Norway. So the Akureyrians started to eat seaweed in greater quantities than ever before. A year after the ending of WWII the first cases of Akureyri disease started to show up; it affected mainly children over six years of age; it affected the Akuryrians who lived in the town and who didn't have any land on which to grow food; Akuryrians who lived on farms hardly got this disease; 10% of the population became ill, some died; some developed Parkinson's disease and became permanently confined to wheelchairs. Again researchers looked for a viral cause but failed to find one. The only explanation is the eating of seaweed.