"How the pleasing plague had stolen on him"

A slight modification of a line from a poem/song by William Whitehead (1715-85) entitled 'The Je ne scai Quoi: A song'.

 

Yes, I'm in love, I feel it now,
And Caelia has undone me;
And yet I'll swear I can't tell how
The pleasing plague stole on me.

                                                               

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