I have already described how my interest in the cause of multiple sclerosis (MS) arose when I first emigrated to South Africa. MS is the most common disabling neurological disorder of young adults in the western world. It seldom occurs before the age of fifteen or sixteen and reaches its peak prevalence in persons in their early thirties. The disorder is more common in women than men, afflicting about three females for every two males, and it is typically a disease of exacerbations and remissions. The nineteenth-century neurologist Jean-Martin Charcot, of the Salpêtrière Hospital in Paris, first gave a good medical account of the disorder in 1857 when he showed that its victims had plaques, or patches of hardening, scattered throughout their brains and spinal cords. He called the disorder sclerose en plaques. The plaques are caused by the loss of myelin, the fatty material that makes up the sheath covering the nerve fibre. When the myelin sheath breaks down, conduction along the nerve fibre is disrupted. It can be compared to the insulation around a telephone wire.

Often the first symptoms of MS are caused by patches of demyelination in the optic nerves, resulting in a blurring of vision in one or both eyes (known as optic neuritis). This visual disturbance may last for a few days or up to six weeks, after which the person's vision generally returns to normal. 35 per cent of men and 75 per cent of women who have an attack of optic neuritis will then go on to develop MS during the ensuing fifteen years. Because the plaques of MS occur anywhere in the central nervous system, they can cause sensory symptoms, such as loss of sensation in any part of the body, loss of muscle power, or ataxia – uncoordination of movement. When the ataxia is severe, it may result in what is known as ‘scanning speech’, a slow enunciation, with a tendency to hesitate at the beginning of a syllable or word.