Brain parasites.

The smoke got into your nose. Burnt milk, not pleasant, yet somehow poignant, colored with remembrances of all your mother's cooking experiments. Folly starts out being pitied, disliked, and then falls into a habit that you get used to without even realizing it, attaching itself to neurons in your brain (a host-parasite relationship, all the four key points covered, check); surviving and growing by evoking nostalgia from unrelated artifacts, years away from the source; sending you down a spiral of nostalgia, nary a care for where you are and whether it's appropriate to turn glassy eyed at a plate of burnt pudding at six in the morning in a seedy diner, while you're gritty with cold and the dust of an unknown city and a night spent in strange hands.

You imagine the neural parasite growing fatter in your brain.

And the smell just wont go.