People infected with Lyme Disease often present with an erythema migrans ("migrating redness") rash. Most often, these rashes are in the shape of a bulls-eye.
Presumably, this is a reaction that many of our bodies have to the Borrelia burgdorferi pathogen.
Lyme disease also sometimes presents with different-looking rashes, and other pathogens also sometimes produce a bulls-eye-looking rash. CDC Link on Lyme Disease Rashes and Look-alikes.
What causes the bulls-eye shape of these rashes?
Thought process: A gradient of redness from the site of the tick bite makes sense--an immune response might be strongest at the bite site--but I'm still stumped on why there would be a spot of redness surrounded by a no-rash-ring surrounded by a rash-ring.
Note: If I remember correctly, external links to images are usually frowned upon. Although the images linked in this post aren't super graphic, I decided to make them external links for the sake of anyone who is sensitive to medical images.