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 psychiatrist is a doctor with specific training in the diagnosis and treatment of mental illness. He or she can not only provide the counseling necessary to both diagnose and treat a patient, but can also prescribe medication when needed. In some cases, a psychiatrist will only provide the medication and the counseling will be provided by another healthcare specialist, like a certified counselor or psychologist. Like other doctors, psychiatrists employ diagnostic tools like CT scans and MRI in order to observe the structure and function of a patient's brain. Once a diagnosis is made, these specialists may use behavior or cognitive therapy in order to address the patient's condition, or a multitude of other types of therapy, in conjunction with or in place of medication.