A pediatrician is a doctor who specializes in the regular care of children, as well as the diagnosis and treatment of illness in children. Young patients are often more complicated to treat because they are still growing and developing.
While pediatricians may sub-specialize in specific therapy areas like oncology, surgery, ophthalmology, and anesthesiology, in general, pediatricians provide services like vaccinations, health exams, and treatment of common ailments and injuries. In addition, pediatricians are trained to handle the complex emotional and behavioral issues faced by children, especially during puberty.
Pediatricians normally see their patients from birth until the age of 18, although some may agree to treat patients into their early 20s, if requested.

Hospitalists are physicians who specialize in the care of patients in the hospital. The majority of hospitalists are board-certified internists and have completed the same training as other internal medicine doctors including medical school, residency and board certification examination.
Hospitalist activities include patient care, teaching, research, and leadership related to hospital care. They have more expertise in caring for complicated hospitalized patients on a daily basis since, unlike other specialists or primary care doctors, they spend most of their day in the hospital.
They often coordinate the care of their patients and act as the central point of communication among the different doctors and nurses involved in the patient's care. They are also the main physician for family members to contact for updates on a loved one.

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.

Bariatric medicine focuses on the causes, prevention and treatment of obesity. Bariatric specialists are trained in medically supervised weight loss and the management of obesity as a chronic condition. These specialists are skilled in various disciplines that help patients lose weight and obtain optimal health.
The field encompasses dieting, exercise and behavioral therapy, anti-obesity medication, pharmacotherapy and surgery. Bariatric specialists might be from surgical backgrounds and perform obesity-related operations or they might be nutrition and hormone focused, prescribing diet and exercise plans as well as weight loss medications.
Patients defined as obese or having unhealthy Body Mass Indices (BMI) have much greater risk of heart disease, diabetes, asthma, sleep apnea, hypertension, many types of cancer and chronic musculoskeletal problems.
One subspecialty of bariatrics is the focus on the correlation between obesity and mortality.