The genes people inherit contribute to their susceptibility for developing an autoimmune disease. Certain diseases such as psoriasis can occur among several members of the same family. This suggests that a specific gene or set of genes predisposes a family member to psoriasis.
Motor neurone disease occurs when specialist nerve cells (motor neurones) in the brain and spinal cord progressively lose their function. It's not clear why this happens. In most cases, a person with motor neurone disease won't have a family history of the condition. This is known as sporadic motor neurone disease.
One or two people in every 100,000 people in the UK have this form of motor neurone disease, around 5,000 people in total. It is rare to develop the condition under the age of 40, as Prof Hawking did. It usually begins between the ages of 50 and 80. Progression varies and about seven in ten people die within five years of diagnosis, with only one in ten living for more than ten years.