incurable and infectious malady

“As you will," said the barber; "but what are we to do with these little books that are left?"

"These must be, not chivalry, but poetry," said the curate; and opening one he saw it was the "Diana" of Jorge de Montemayor, and, supposing all the others to be of the same sort, "these," he said, "do not deserve to be burned like the others, for they neither do nor can do the mischief the books of chivalry have done, being books of entertainment that can hurt no one."

"Ah, senor!" said the niece, "your worship had better order these to be burned as well as the others; for it would be no wonder if, after being cured of his chivalry disorder, my uncle, by reading these, took a fancy to turn shepherd and range the woods and fields singing and piping; or, what would be still worse, to turn poet, which they say is an incurable and infectious malady."

Miguel de Cervantes’ Don Quixote (1605), Chapter VI: “Of the diverting and important scrutiny which the curate and the barber made in the library of our ingenious gentleman”