An estimated 476,000 Americans are infected each year with Lyme disease, a condition causing a wide range of symptoms that include fever, rash, and joint pain, as well as effects on the central nervous system and heart. Though it's common knowledge that Borrelia burgdorferi—the bacteria that causes the disease—enters the body through the bite of an infected deer tick, how the bacteria manages to migrate from that bite into a person's bloodstream has not been clearly understood.