There is a type of complete colorblindness, achromatopsia, where people do not have functional cones in their eyes and are almost blind in sunlight because of the sensitivity of the rods. Achromatopsia, unlike red-green colorblindness, is very rare. The island of Fuur and the island of Pingelap both had large numbers of people suffering from this congenital achromatopsia. Only Pingelap, in the south Pacific, still has large numbers of achromatopes. The author visited Pingelap with a physiologist who is an achromatope himself and a friend who is an ophthalmologist. He writes a pretty interesting travel biography of their trip in addition to talking about the examinations and interviews with the achromatopes and how such large numbers of them cope with limited vision. He has a strong interest in their compensations and alternate ways of perceiving the world. Achromatopes may have an advantage in such things as night diving to capture fish because of their dark vision is strong.
The second part of the book is about traveling to the island of Guam to visit a colleague who treats neurological disorders. This colleague, Dr. John Steele, wants the author to see the cases of lytico and bodig, which are prevalent in Guam. Dr. Steele has been treating and looking for reasons for lytico and bodig and these diseases have symptoms similar to those of patients the author treats for post-encephalitic disorders. Both lytico and bodig, while declining, are believed to have had the same cause sometime in the past on Guam. The candidates are explained with pros and cons for each and there is discussion of similar diseases in other places.