The study consists of two sessions: the screening session and the experimental session. During the initial phone screening, individuals who contact the study team will be given information about the study, including a description of all study-related procedures. If interested, participants will then be taken through a phone screen interview that will ask them to report demographic information (date of birth, race, ethnicity), height and weight, and list of medical conditions and current medications, surgical history within the past year, amount of weekly consumption of alcohol, tobacco products, and illicit substances, viral or bacterial infections within the past 3 months, recent vaccines received, and current participation in other research. The standard MRI Safety form will be administered to ensure participants do not have ferromagnetic implants or other contraindications to MRI (e.g. claustrophobia).A self-report will also be administered to ascertain that potential fibromyalgia participants meet 2016 American College of Rheumatology (ACR) criteria for fibromyalgia. The phone screen is intended to reduce participant burden by identifying exclusion criteria without the need to attend at UAB. Participants who meet initial screening criteria will be invited to attend an in-person screening visit.

During the screening visit, participants will go to the Clinical Research Unit (CRU) on the 15th floor of Jefferson Tower. Participants will be asked to fast prior to this visit. Participants will provide written informed consent before any study procedures are initiated. Participants will provide a urine sample for pregnancy testing. Vital signs (blood pressure, heart rate), aural temperature, height and weight will be measured and recorded. Participants will undergo 12-lead electrocardiogram and provide a maximum of 60cc of blood into labeled blood collection tubes for screening tests. A tender point exam will be administered to assess fibromyalgia symptomatology. The procedure uses a pressure algometer, a hand-held instrument with a small rubber tip that is applied to the participant's skin over standardized regions of the body (occipital, low cervical, upper trapezius, supraspinatus, anterior second rib, lateral epicondyle, gluteal, greater trochanteric, or medial knee). Pressure is slowly increased at a constant rate until the participant reports the sensation changing from one of pressure to one of pain or discomfort. Participants will complete a demographics form , Brief Pain Inventory, Fibromyalgia Impact Questionnaire, Multidimensional Fatigue Inventory, and Multiple Abilities Symptom Questionnaire. The blood will be tested by the UAB Hospital Labs to ensure that the participant meets the study criteria. The ECG will be forwarded to the study physician who will assess for evidence of conduction abnormalities or of ischemia. If the blood tests or ECG are abnormal, the study physician will determine if the participant should be advised to see their PCP. Following availability of all results from the screening visit, participants will be contacted via phone and informed of their eligibility to attend the experimental session.

During the experimental session, the administration of endotoxin, brain imaging, and related procedures will be performed at the CRU. No fasting is required prior to this visit, and participants will be asked to eat breakfast prior to arrival and will be provided with meal suggestions. Baseline vital sign measurements consisting of resting blood pressure and pulse rate will be measured and recorded within 60 minutes prior to the start of imaging. ECG and pregnancy tests will be repeated and participants will be excluded from participation in the session if conduction abnormalities are found (same criteria as used during screening) or if participants are found to be pregnant. Participants will undergo MRI imaging for approximately 1 hour. Participants will leave the scanner and complete questionnaire measures assessing participant's current level of fatigue, pain, and mood symptoms. Grip strength will also be assessed. A low dose of endotoxin (0.3ng/kg) will then be administered by the study nurse via intravenous infusion. Vital signs will be periodically monitored for the rest of the session. Participants will remain in the laboratory for three hours before being placed into the MRI scanner to complete a second, identical, imaging session. The self-report questionnaire measures and grip strength will be repeated every hour following endotoxin administration in order to quantify changes in symptoms due to LPS. To avoid participant discomfort and to optimize study integrity, a standardized meal of approximately 600-kcal will be provided to each participant between the two MRI sessions, as well as a later snack of approximately 200-kcal. The meal is standardized to caloric quantity and timing to offset any effects of food intake on the immune response.