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 <WSD>malignant</WSD> diseases. […]” 
Meaning of the word marked with <WSD> tags:  Tending to produce death; threatening a fatal issue.