What term describes the system for the diagnosis, treatment, and prevention of illness?

The term that encompasses the diagnosis, treatment, and prevention of illnesses is 'medicine.' This field is primarily concerned with the overall management of health and medical conditions, involving a variety of practices aimed at maintaining or restoring health. Medicine combines multiple disciplines and approaches, including the use of pharmaceuticals, surgical procedures, behavioral therapies, and preventive measures to enhance patient outcomes.

While 'healthcare' also relates to the broader system that includes activities performed to promote health, prevent illness, and treat patients, it is a more encompassing term that includes various professionals, services, and systems beyond just medical practice. 'Pharmacy' is focused on the preparation, dispensing, and appropriate use of medications, thereby playing an essential role in medicine but not encompassing the entirety of diagnosis and treatment. 'Pathology,' on the other hand, refers specifically to the study and diagnosis of disease through the examination of tissues, cells, and organs, which is a crucial part of medicine but does not represent the full spectrum of diagnosis, treatment, and prevention.

Therefore, 'medicine' is the most fitting term that directly describes the systematic approach to managing illness and maintaining health.