There is no one at the present time who writes like Oliver Sacks. Living proof that there need be no impenetrable divide between the arts and the sciences, this Clinical Professor of Neurology at one of New York’s finest medical schools writes of disease and disability with profound empathy and impressive erudition. He is a superb clinician who can take a seemingly arid and obscure medical condition, and convert it into a moving, personal odyssey, a testament of tenacity, courage and will. He has transformed the clinical case history, the foundation stone of medical education, into a literary art form.

In his latest book, Sacks moves from the painstaking and intimate description of sick or disabled individuals to the examination of groups who have been marked by some defect or deficiency. Islands, particularly far-flung and isolated ones, given the propensity for in-breeding, are fertile places for the development of genetically transmitted disorders. In the tiny Pacific atoll of Pingelap, for example, there is a community of islanders born totally colour-blind. The first children with the genetic mutation were born in the 1820s. Today, a third of the islanders are carriers of the gene, and out of some eight hundred people, fifty-seven are colour-blind. Elsewhere in the world, the incidence of achromatopsia is less than one in thirty thousand. In Pingelap, it is one in fourteen.

Sacks, accompanied by an eye specialist and a physiologist who is himself colour-blind, visited the island and explored the impact of the condition and its absorption into the island’s myths and culture. Somewhat to his surprise, he discovered that the deficiency is well-tolerated and causes little hardship. There are, however, widespread misapprehensions and anxieties about what colour-blindness might mean – that it might be progressive and lead to total blindness, that it might be accompanied by retardation, madness, epilepsy or heart trouble, that it might be caused by carelessness during pregnancy or transmitted by some form of contagion. In an absorbing account of the mythology and meaning of the deficiency, Sacks explores the significance of colour, the extraordinary ability of the colour-blind to distinguish the subtlest shades of grey, their skill in operating at night, and the manner in which the condition becomes absorbed into the culture and fabric of the island society.

On another Pacific island, Guam, there is a mystifying neurodegenerative paralysis which has been endemic for one hundred years. Lytico-bodig, as the disease is called by the people of Guam, presents itself in different ways – sometimes as ‘lytico’, a progressive paralysis which resembles motor neurone disease and which causes the sufferer to waste away and die; sometimes as ‘bodig’, a condition resembling parkinsonism, occasionally with dementia. The paralysis is slowly disappearing, but there are still hundreds of sufferers. Despite years of research and study, the disease remains unexplained. Sacks, drawing on his own experience of post-encephalitic disorders, embarked on a search for the possible cause. The main suspect was the primitive cycad trees which grow all over the island and which provide the islanders with a popular flour made from its seeds.

Yet the cycad theory proves problematic. First, there are no known examples outside Guam of a chronic human illness imputed to the use of cycads, despite their wide and long use throughout the world. Secondly, the period of decades which elapses between exposure to the cycads and the onset of lytico-bodig has no precedent in poisonings of the human nervous system. All known neurotoxins act immediately or within a few weeks. Sacks pondered these mysteries while he visited and consoled many of the island sufferers. He confesses to feeling drained by seeing many of them in the terrible final stages of the disease, incurable and dying, cared for with enormous dedication by an order of nuns.

The result is a long, moving essay – part clinical, part botanical – which weaves together Sacks’s fascination with medicine and his love and knowledge of trees and plants. At times he is speculative, at times reverential, yet he never strikes a false note, never parades his learning, never pontificates, never postures. There is something elegiac in the writing as he mourns the passing of complex myth and tribal explanation while valuing the information and the control afforded by the rise of scientific knowledge and understanding. Throughout, he remains an unashamed romantic, but he is a romantic, with an ophthalmoscope, visual test cards and a bag full of correcting spectacles. He can understand, even admire, the myth while slowly slaying it with a simple scientific fact. Neither island appears the worse for his having visited it. Would that the same could be said of many who went before him.