Borrelia burgdorferi is a spirochete bacterium that multiplies in ticks and is spread to mice and white-tailed deer by the bite of an infected Ixodes scapularis tick. Humans are an accidental host in this life cycle when infected by ticks. The disease that arises after infection, Lyme disease, is named after the town in Connecticut where the disease was identified after an outbreak of juvenile rheumatoid arthritis in children occurred. One hallmark of the infection is a rash at the site of the tick bite that looks like a bull's-eye.

This image is a dark-field microscopic image of the spirochete taken from a culture of the organism in the laboratory. The dark-field microscopy allows visualization of these tightly curled spiral bacteria that do not stain well with conventional bacterial stains.