You say thank you or something related to that.
"tan bonito" does not make part of the sentence. The sentence is describing a girl, so instead it should say;
"tan bonita"
How quickly the plague spread
by talking about a swift change in time where death and spreading occurs its is very clear that this is about how quickly the plague spreads
I''m thinking either Metaphor, but I'm not quite sure.. sorry if this didn't help much.