Inclusion Criteria:
* Inclusion criteria for pre-pandemic onset ME/CFS cohort:
* 18-70 years of age
* At least 8th grade education
* Onset of the following symptoms prior to July 2019 (Pre-SARS-CoV-2 pandemic) and meeting 2015 IOM diagnostic criteria for MECFS:
* Substantial reduction in functioning and persistent and profound fatigue for at least 6 months. The person is unable to undertake the same level of activity before they became - . Their fatigue is not the result of significant exertion and is not substantially alleviated by rest.
* Post-exertional malaise.
* Unrefreshing sleep
* At least one of the following two symptoms:
* Cognitive impairment (difficulty with memory, finding words, understanding etc).
* Orthostatic intolerance (symptoms like palpitations, sweating, dizziness, nausea when standing that are reduced or eliminated when lying down).
Inclusion criteria post SARS-CoV-2 onset ME/CFS:
* 18-70 years of age
* At least 8th grade education
* Previous suspected, probable, or confirmed SARS-CoV-2 infection, as defined by the World Health Organization:
* Suspected case of SARS-CoV-2 infection
* Three options, A through C:
Meets the clinical OR epidemiological criteria.
* Clinical criteria: Acute onset of fever AND cough (influenza-like illness) OR Acute onset of ANY THREE OR MORE of the following signs or symptoms: fever, cough, general, weakness/fatigue, headache, myalgia, sore throat, coryza, dyspnea, nausea, diarrhea, anorexia.
* Epidemiological criteria: Contact of a probable or confirmed case or linked to a COVID-19 cluster; or B. Presents with acute respiratory infection with history of fever or measured fever of ≥ 38°C; and cough; with onset within the last 10 days; and who requires hospitalization) C. Presents with no clinical signs or symptoms, NOR meeting epidemiologic criteria with a positive professional use or self-test SARS-CoV-2 antigen-Rapid Diagnostic Test.
* Probable case of SARS-CoV-2 infection, defined as meets clinical criteria above AND is a contact of a probable or confirmed case or is linked to a COVID-19 cluster.
* Confirmed case of SARS-CoV-2 infection - Two options, i. and ii.:
* A person with a positive nucleic acid amplification test, regardless of clinical criteria OR epidemiological criteria; or
* Meeting clinical criteria AND/OR epidemiological criteria (See suspect case A). With a positive professional use or self-test SARS-CoV-2 Antigen-Rapid Diagnostic Test.
* Post SARS CoV 2 onset of the following symptoms, meeting 2015 IOM diagnostic criteria for ME/CFS:
* Substantial reduction in functioning and persistent and profound fatigue for at least 6 months. The person is unable to undertake the same level of activity before they became - - Their fatigue is not the result of significant exertion and is not substantially alleviated by rest.
* Post-exertional malaise.
* Unrefreshing sleep.
* At least one of the following two symptoms:
* Cognitive impairment (difficulty with memory, finding words, understanding etc).
* Orthostatic intolerance (symptoms like palpitations, sweating, dizziness, nausea when standing that are reduced or eliminated when lying down).
* Given that our application focuses on MECFS patients with neuropsychiatric symptoms the investigators will require participants to report \>1 symptom with at least one of the symptoms being neuropsychiatric in nature (e.g., brain fog, anxiety, depression, headaches).
Exclusion both groups (pre-pandemic onset ME/CFS and post SARS-CoV-2 onset ME/CFS):
* History of Axis I psychiatric disorders (including alcohol and drug dependence) except for mood disorders which developed after onset of ME/CFS symptoms.
* Severe or unstable medical condition that would interfere with physical participation.
* Neurological disorders, such as stroke, epilepsy, multiple sclerosis, Parkinson's disease, or dementia.
* History of severe head injury that resulted in persistent traumatic brain injury sequelae.
* Any condition that is contraindicated for the MRI environment (e.g., metal in the body, pacemaker, claustrophobia)
* Currently pregnant.
* Lack of English proficiency (to ensure validity in verbal outcome measures)
* Active or recent (within 3 months) substance misuse (which is known to affect BBB permeability) assessed by clinical interview and toxicology (nicotine/cannabis allowed)
* History of psychosis
