Four people with multiple sclerosis tell their stories about the condition. MS affects the central nervous system and is most common in the Northern Hemisphere.
Dan O'Brien, his wife Karen Sherwood and his cousin Helen McDougall live in Canada. Catherine Hannah, 37, is from Orkney and was first diagnosed with MS as an 18-year-old university student.
There is more about multiple sclerosis and the apparent Scottish link in Elizabeth Quigley's television investigation, "Scotland's Hidden Epidemic: The truth about MS", which will be broadcast on BBC One Scotland at 2245 BST on Wednesday.