On the Pacific Coast, the bacteria are transmitted to humans by the western black-legged tick . The western black-legged tick is much smaller than common dog and cattle ticks. In their larval and nymphal stages, they are no bigger than a pinhead. Adult ticks are slightly larger. Larvae and nymphs take in spirochetes as they feed on infected mice.
Infected larvae molt to infected nymphs and infected nymphs molt to infected adults.
More cases of people infected with Lyme disease are the result of the feeding of infected nymphs rather than infected adults. Ticks will attach anywhere on the body, but prefer body creases such as the armpit, groin, back of the knee, and nape of the neck. Most cases of Lyme disease occur between late spring and early fall when people are outdoors and ticks are most active. As they feed, their bodies slowly enlarge. Most B. burgdorferi infections are thought to result from exposure to infected ticks during property maintenance, recreation, and leisure activities.