Functional neurological disorder/illness is a disorder of the nervous system which creates all kinds of medically unexplained symptoms such as limb weakness, sinus troubles, IBS, dizziness, anxiety and depression etc. A lot of the time symptoms such as spots or flu like symptoms can appear out of nowhere and not have a cause.
The NHS website describes it as such; Functional Neurological Disorders (FND’s) is the name given for symptoms in the body which appear to be caused by problems in the nervous system but which are not caused by a physical neurological disease or disorder. Health professionals sometimes call these disorders ‘medically unexplained’, psychosomatic or somatisation. We prefer the term ‘functional’ which just means that the body is not functioning quite as it should.
A link to the following website outlines this disorder.