An Oncologist is a physician who focuses on the diagnosis and treatment of different cancers. This physician has extensive knowledge of the different signs and symptoms of cancer, as well as the various methods of treatment.
Oncologists diagnose cancer through methods such as biopsy, endoscopy, X-ray, blood tests, ultrasound, and different forms of nuclear medicine. They treat cancer through surgery, chemotherapy, radiotherapy, hormone therapy, or antibody treatments.
If it is determined that a cancer cannot successfully be treated, oncologists then focus on providing palliative care, the use of pain medication to make a dying person more comfortable.

An internist is a physician who focuses on the diagnosis and treatment of conditions that affect the adult population—both acute and chronic.
These doctors are often who adults see as their primary physicians because they treat a broad range of illnesses that do not require surgical or specialist interventions. They also work to help a patient maintain optimal health in order to prevent the onset of disease.
In addition to treating the common cold and flu, internists also treat chronic diseases like diabetes and heart disease.

A physiatrist is a doctor who specializes in the rehabilitation and physiological treatment of patients with an illness or injury that affects movement.
These specialists have extensive knowledge of the nerves, muscles, bone, and brain. Physiatrists are also experts in pain medication.
Some common conditions that physiatrists treat are rheumatoid arthritis, neurological and spinal disorders and injuries, chronic pain disorders, like fibromyalgia, and musculoskeletal issues, like broken bones and torn muscles.
These physicians also often coordinate a team of other specialists in order to maximize the patient's recovery, such as occupational therapists, speech therapists, neurologists, orthopedists, and counselors.