The hippocampus is a region of the brain important for consolidation of information from short-term memory to long-term memory, spatial navigation, and mood. Its name derives from the Greek word for seahorse, in that it mimics a seahorse in size and shape.
A TED talk by Sandrine Thuret reported that the hippocampus was able to generate new nerve cells, a process called neurogenesis, to replace older hippocampus nerves. Brian neurogenesis was once believed to not exist in adults. If the hippocampus can not generate new cells because of illness or medication, poor memory and depression may result. 
Hippocampus and Its Role in ME/CFS
The hippocampus is shown to be damaged or atrophied in ME/CFS and fibromyalgia. Sleep disturbances common in ME/CFS and FM can, in turn, further impact the hippocampus and initiate a vicious circle of increased disability.
In 2014, researchers in Japan, at the RIKEN Center for Life Science Technologies, in collaboration with Osaka City University and Kansai University of Welfare Sciences, used functional PET imaging to show that levels of inflammation of the nervous system, are higher in patients with chronic fatigue syndrome than in healthy people. The researchers found that inflammation in certain areas of the brain—the cingulate cortex, hippocampus, amygdala, thalamus, midbrain, and pons—was elevated in a way that correlated with the symptoms.
- http://www.ncbi.nlm.nih.gov/pubmed/26804593 "The role of the hippocampus in the pathogenesis of Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS)," JM Saury, Medical hypotheses,Med Hypotheses. 2016 Jan;86:30-8. doi: 10.1016/j.mehy.2015.11.024. Epub 2015 Nov 27
- http://www.riken.jp/en/pr/press/2014/20140404_1/ “Neuroinflammation in patients with chronic fatigue syndrome/myalgic encephalomyelitis: a 11C-(R)-PK11195 positron emission tomography study,” The Journal of Nuclear Medicine, vol.55, No.6, 2014, DOI: 10.2967/jnumed.113.131045
- http://www.ncbi.nlm.nih.gov/pubmed/8879459 "EEG biofeedback as a treatment for chronic fatigue syndrome: a controlled case report," Behavior Medicine,1996 Summer;22(2):77-81