Also sometimes referred to as intensivists, critical care specialists are physicians with specialized training in the diagnosis and management of life-threatening conditions. Some of these conditions affect vital organs like the heart and lungs, those that make breathing difficult or impossible, and those that affect entire organ systems, like the renal system.
Critical care specialists are typically found in a hospital's intensive care unit where they monitor patients with life-threatening conditions and make determinations as to the best course of treatment.

A pulmonologist is a physician who specializes in the diagnosis and treatment of conditions related to the lungs and respiratory tract.
These specialists are similar to critical care specialists in that their patients often require mechanical ventilation to assist their breathing.
Pulmonologists diagnose and treat patients with conditions such as asthma, cystic fibrosis, asbestosis, pulmonary fibrosis, lung cancer, COPD, and emphysema. Exposure and inhalation of certain toxic substances may also warrant the services of a pulmonologist.
Some of the tools and tests pulmonologists use to diagnose a patient are a stethoscope in order to listen for abnormal breathing sounds, chest X-rays, CT scans, blood tests, bronchoscopy, and polysomnography.

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.

An endocrinologist is a physician with extensive training in understanding, diagnosing and treating conditions related to the endocrine system. The endocrine system regulates the balance of hormones.
Conditions of the endocrine system involve an over-abundance, or deficiency of a certain hormone. While there is a range when it comes to the amount of a hormone that is deemed normal in a human, these specialists determine whether a person's amount of hormone is indicative of a health concern. Two conditions this specialist might treat are diabetes and obesity.