Objective The primary objective is to explore the clinical and biological phenotypes of post-infectious myalgic encephalomyelitis/chronic fatigue syndrome (PI-ME/CFS). The secondary objective is to explore the pathophysiology of fatigue and post-exertional malaise (PEM). Study population Up to 346 persons will be enrolled as part of this protocol. Up to 150 persons aged 18-60 will be part of 3 study groups: 50 ME/CFS patients whose fatigue began after an infection, 50 non-fatigued participants with a documented history of Lyme disease exposure and treatment, and 50 healthy volunteers. The study has a target of completing all study procedures on 20 enrolled participants in each group. Up to an additional 176 persons reporting a community diagnosis of ME/CFS will be enrolled into focus groups to discuss the experience of post-exertional malaise. Up to an additional 10 healthy volunteers and 10 ME/CFS patients may be enrolled to refine the protocols electrophysiological and neuroimaging techniques. Design This is a single-center, exploratory, cross-sectional study of PI-ME/CFS. Participants will have a phenotyping visit, which will encompass a 2-5 day long inpatient admission at the NIH Clinical Center. Case status for ME/CFS participants will be determined after the phenotyping visit by a case adjudication process utilizing an expert physician committee and published guidelines. Adjudicated participants meeting inclusion criteria will be invited back to participate in an exercise stress visit, which will encompass a 5-10 day long inpatient admission. Detailed subjective and objective measurements and biological specimens will be serially collected before and up to 96 hours after a peak exercise test capable of inducing post-exertional malaise during this visit. All procedures will be completed on all three study groups to allow for optimal inter-group comparisons. Outcome Measures The primary purpose of this protocol is to perform exploratory analysis of collected samples for the generation of new hypotheses regarding ME/CFS. The types of analyses to be performed will be wide ranging. Planned areas of focus include: 1. Characterization of the immune system and inflammatory signaling in collected samples at baseline and following maximal exercise exertion. 2. Characterization of the pattern of microbiome in collected samples at baseline and following maximal exercise exertion. 3. Characterization of bioenergetics, autonomic, and metabolic function in collected samples at baseline and following maximal exercise exertion. 4. Characterization of physical and cognitive fatigue using functional magnetic resonance imaging and transcranial magnetic stimulation at baseline and following maximal exercise exertion. 5. Characterization of neurocognition at baseline and following maximal exercise exertion. 6. Characterization of brain function and connectivity at baseline and following maximal exercise exertion. 7. Characterization of autonomic function at baseline and following maximal exercise exertion. 8. Characterization of gene expression profiles in collected samples at baseline and following maximal exercise exertion.