Sentence:  “[…] the awfully hearty sort of Christmas cards that people do send to other people that they don't know at all well. You know. The kind that have mottoes.... And then, when you see [the senders], you probably find that they are the most melancholy old folk with malignant <WSD>diseases</WSD>. […]” 
Meaning of the word marked with <WSD> tags:  An abnormal condition of a human, animal or plant that causes discomfort or dysfunction; distinct from injury insofar as the latter is usually instantaneously acquired.