Antidiuretic hormone

Vasopressin, also named antidiuretic hormone (ADH), arginine vasopressin (AVP), or argipressin is a hormone that causes the kidneys to reabsorb water and return it to the circulation, thus controling the concentration of urine.

ME/CFS
According to Dr. David Kaufman formerly of the Open Medicine Institute, diagnostic biomarkers for ME/CFS include low natural killer cell function, low vasopressin levels, mutations of the MTHFR gene, and abnormalities of the Hypothalamic-pituitary-adrenal axis (HPA).

Note that by this definition, the infectious agent involved does not define the disease; indeed many people have many reactivated pathogens only revealed upon careful and reliable testing, including, herpesviruses, parvovirus B19, lyme, babesia, bartonella, etc.