- Why CDC links to IDSA Guidelines

	Chart: Life cycle of Lyme disease ticks

Blacklegged ticks live for two years and have three feeding stages: larvae, nymph, and adult. When a young tick feeds on an infected animal, the tick takes the bacterium into its body along with the blood meal. The bacterium then lives in the gut of the tick. If the tick feeds again, it can transmit the bacterium to its new host. Usually the new host is another small rodent, but sometimes the new host is a human. Although adult ticks often feed on deer, these animals do not become infected. Deer are nevertheless important in transporting ticks and maintaining tick populations.

The natural transmission cycle for Lyme disease is not found in all parts of the United States. For example, in southern United States, blacklegged ticks can take a blood meal from a lizard instead of a small rodent. This interrupts the natural transmission cycle and lowers the chance of Lyme disease transmission to humans. In some mid-western United States, the climate is too dry and hot for blacklegged ticks to live. The chance of Lyme disease transmission in these regions is very low. Even in areas where Lyme disease is common, not all black-legged ticks contain the Borrelia burgdorferi bacterium. The percentage may vary widely from place to place, but generally less than half of ticks are infected.