It is thought that around 250,000 men, women and children in the UK have M.E., although it is most common in women aged 25-50.
There is a strong possibility that someone you know is affected by the illness.
If you work in a large organisation, 1 in 250 members of staff is likely to have M.E.
M.E. is defined as a neurological illness by the World Health Organisation.
NHS guidelines say the symptoms can be as disabling as multiple sclerosis, lupus, rheumatoid arthritis or congestive heart failure.
However, levels of severity vary from mild and functioning to housebound or bedbound.
As M.E. is a fluctuating illness, symptoms also vary day-to-day.
Find out more.