Characterizing Fatigue Experienced by Cancer Patients Receiving Primary Treatment and Cancer Survivors

Background: \- Fatigue is a common side effect of cancer and its treatment. Up to 96% of patients with cancer have fatigue. The cause of fatigue in people with cancer or those receiving treatment is poorly understood, making fatigue hard to treat. More research is necessary to understand why cancer patients experience fatigue. Previous research has shown that fatigue may be caused by a number of factors, including problems with the immune system response of the body while undergoing cancer treatment. The researchers of this study are trying to understand how the immune system and other chemicals of the body change while a person receives cancer treatment and how these changes play a role in causing fatigue. The researchers are also interested in how a person s quality of life is affected by fatigue. Objectives: \- To study fatigue in individuals who have cancer, are receiving cancer treatment, or have completed cancer treatment. Eligibility: * Individuals at least 18 years of age. * Receiving cancer treatment, OR completed primary treatment for cancer (e.g., cancer survivors), OR on active surveillance for localized cancer. Design: * This study involves an initial screening visit and up to three outpatient visits. * Participants will be screened with a medical history review (can be done virtually) and blood tests. A physical exam might also occur. * Participants undergoing cancer treatment that has a clear completion date, will have the following visits: * Before the start of treatment. * At the end of treatment. * At least 3 months after treatment ends. * Participants not receiving treatment, are cancer survivors, or undergoing cancer treatment that does not have a clear completion date, will have up to 3 visits: * Before the start of treatment or whenever you begin the study. * At least 3 months after your first visit. * At least 3 months after your second visit. * At each study visit, participants will complete the following tasks (some of t