The concept of frailty historically has been the object of a lot of debate among those who work in the field of elderly health, with differences between epidemiological and clinical outlook.
For geriatricians the frail have for a long time been people with multiple health problems and often with an already advanced disability, such as those that can be found in hospitals or nursing homes. Epidemiological Revolution introduced by Linda Fried, a most prominent geriatrician and epidemiologist that I have already quoted in the post "Thinking differently in healthy aging", changed the paradigm in early 2000.
In plain language, she indicated as "frail" something at risk of breaking, not already "broken", as in the case of people with advanced disabilities that we mentioned. So a frail person, in this view, is a person with apparent good health, and even without a disability, which has reduced physiological reserves of different organs and systems that makes them particularly susceptible to descend towards disability in case of injuries of a different nature (clinical, such as illness, or social, as a widow, etc). Detection of frailty in this sense is mainly based on measures of physical or cognitive performance, with a clear objective of prevention, since it is proven that the frailty is reversible and targeted interventions (exercise, nutrition, geriatric assessment) can prevent disability.