The man opposite you on the train has one leg in plaster and one arm in a sling. Each time he moves, he grimaces with pain. Few of us could fail to have a degree of sympathy for his suffering. The apparently normal man seated next to him may be experiencing excruciating burning and stabbing pain in his legs and feet as a result of his MS. Who is to know, though? There is nothing to see to explain his pain. Neuropathic pain is truly an invisible illness.
Source MS Trust: