ABANDONED. Fantasy Children's/YA book about creatures modifying humans they come into contact with

[Sarah]

I'm looking for a fantasy/steampunk? (the setting is in a city of that doesn't seem that modern) book I read somewhere between 2010 and 2015 (although it could have been released earlier). It's about this world where creatures can modify human appearances by touching them. The main characters are a boy and a girl, although there's probably more than just them. I don't remember the name of these creatures or the reason of their existence, but when one of the main characters came into contact with one of them, he lost his ability to breathe properly. Someone then gave him a mask that he never takes off during most of the story. Another example is when the girl and boy were running away from one of the creatures, it went through the girl and her eyes turned red as a result. There was also this half human/machine guy whom the main characters came across: at first they thought it was the work of one of the creatures (although he was man-made) so these things can apparently do a lot more than minor changes to the human body. I don't remember much else but I hope it's enough to recognize it! Thank you.

[Lobstergirl, au gratin]

Sarah, are you still looking for this or did you find it?

[Kris]

No response, moving to Abandoned folder.

Sarah (OP) was last active on the site in May 2018.