I had thoughts along these lines: there was a major outbreak of giardiasis (Giardia lamblia infection) in Bergen, Norway in 2004 (an estimated 2500 cases), so this lingering pathogen may now be common in the intestines of a subset of people in Bergen. Giardia lamblia is linked to ME/CFS, as well as autoimmune diseases. Fluge and Mella are based in Bergen, so it occurred to me that their cohort may contain lots of people who developed ME/CFS as a result of Giardia lamblia. More in this post.
I think they've adressed this. I think that zero or perhaps one of their patients got ME from the outbreak.
One of the highest risk groups are people living in the same household as a case of giardiasis because Giardia can be transmitted by the faecal-oral route from person-to-person. Between 35-70% of people infected with Giardia have no symptoms but are still infectious to others. Without treatment, infectivity can continue months resulting in potential infection or re-infection of others.