The College of Physicians of Philadelphia is the oldest private medical society in the United States.

Founded in 1787 by 24 Philadelphia physicians "to advance the Science of Medicine, and thereby lessen human misery, by investigating the diseases and remedies which are peculiar to our country" and to promote "order and uniformity in the practice of Physick," it has made important contributions to medical education and research.

The College hosts the Mütter Museum, a gallery of 19th-century specimens, teaching models, instruments, and photographs, as well as the Historical Medical Library, which is one of the country's oldest medical libraries.

Immunization, or immunisation, is the process by which an individual's immune system becomes fortified against an agent (known as the immunogen).

A vaccine is a biological preparation that provides active acquired immunity to a particular disease.

A vaccine typically contains an agent that resembles a disease-causing microorganism and is often made from weakened or killed forms of the microbe, its toxins or one of its surface proteins.

The agent stimulates the body's immune system to recognize the agent as a threat, destroy it, and keep a record of it so that the immune system can more easily recognize and destroy any of these microorganisms that it later encounters.

Vaccines can be prophylactic (example: to prevent or ameliorate the effects of a future infection by any natural or "wild" pathogen), or therapeutic (e.g., vaccines against cancer are being investigated).

Public health refers to "the science and art of preventing disease, prolonging life and promoting human health through organized efforts and informed choices of society, organizations, public and private, communities and individuals." It is concerned with threats to health based on population health analysis.

The population in question can be as small as a handful of people, or as large as all the inhabitants of several continents (for instance, in the case of a pandemic).

The dimensions of health can encompass "a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity," as defined by the United Nations' World Health Organization.

Public health incorporates the interdisciplinary approaches of epidemiology, biostatistics and health services.

Environmental health, community health, behavioral health, health economics, public policy, mental health and occupational safety and health are other important subfields.