Final answer:
The suffix indicating 'one who treats' or 'a physician' is '-iatrist', as seen in professions like psychiatrist or orthopedist.
Step-by-step explanation:
The suffix that means one who treats or a physician is -iatrist. This suffix is often found in words related to medical specialties focused on treatment, such as psychiatrist or podiatrist. The term geriatrics, for example, was created by Dr. Ignatz Nascher by combining the Greek word geron meaning 'old man' and the suffix -iatrist from iatricos meaning 'medical treatment'. An orthopedist is an example of a physician who treats musculoskeletal system disorders and injuries. The origin of the word 'orthopedics' combines ortho- meaning 'straight' with paed- meaning 'child', but in modern times, orthopedists treat patients of all ages with conditions ranging from congenital defects like spina bifida to age-related conditions.