for the following reasons:

--A person may be weak in health without having a specific disease, or a diagnosis of one (consider the state of medical knowledge at the time);

--IMO, a person who is truly strong and healthy could not bear to live as Mr. Woodhouse does. They might feign illness or weakness on occasion to manipulate -- think of Mary Musgrove or Mrs. Bennet, who only claim to be suffering when it suits them, but can turn instantly well and strong once they've gotten what they want -- but Mr. Woodhouse is very consistent in his invalid mode of life.

--What reason has Mr. Woodhouse for manipulating people? He is rich, he is the head of his family, the head and elder of the society in which he lives. People would pay him plenty of deference in any case. Except:

--I doubt Mr. Knightley, if he suspected Mr. Woodhouse were feigning a weakness he really did not feel, would be much inclined to encourage him in any such behavior. Mr. Woodhouse being the elder, I don't imagine he would confront him with his faults as he confronts Emma with hers, but I don't think he would be so very considerate and patient with him as he is.