The Lyme disease bacterium, Borrelia burgdorferi, normally lives in mice, squirrels and other small animals. It is transmitted among these animals - and to humans - through the bites of certain species of ticks. In the northeastern and north-central United States, the black-legged tick (or deer tick, Ixodes scapularis) transmits Lyme disease. In the Pacific coastal United States, the disease is spread by the western black-legged tick (Ixodes pacificus). Other major tick species found in the United States have not been shown to transmit Borrelia burgdorferi.

Blacklegged ticks transmit Lyme disease
Blacklegged (or deer) ticks (Ixodes scapularis and Ixodes pacificus) can transmit several tick-borne diseases including anaplasmosis, babesiosis and Lyme disease. An adult tick is pictured at left, though it is the smaller nymphal stage ticks which most commonly bite humans.		This image shows the stages and relative sizes of these tick species. Only the blacklegged ticks are known to transmit Lyme disease.
Ticks not known to transmit Lyme disease
Lone star ticks (Amblyomma americanum) have been linked to transmission of ehrlichiosis, tularemia, and southern tick-associated rash illness (STARI). The saliva of these ticks is irritating, and can cause an allergic reaction at the site of the bite
American dog tick (Dermacentor variabilis) as well as the Rocky Mountain wood tick (Dermacentor andersoni) can transmit many diseases including Rocky Mountain spotted fever and tularemia.

Life cycle of blacklegged ticks
Blacklegged ticks live for two years and have three feeding stages: larvae, nymph, and adult. Tick eggs are laid in the spring and hatch as larvae in the summer. Larvae feed on mice, birds, and other small animals in the summer and early fall. When a young tick feeds on an infected animal, the tick takes bacteria into its body along with the blood meal, and it remains infected for the rest of its life. After this initial feeding, the larvae become inactive as they grow into nymphs. The following spring, nymphs seek blood meals in order to fuel their growth into adults. When 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. Most cases of human illness occur in the late spring and summer when the tiny nymphs are most active and human outdoor activity is greatest. Adult ticks feed on large animals, and sometimes on humans. In the spring, adult female ticks lay their eggs on the ground, completing the life cycle. Although adult ticks often feed on deer, these animals do not become infected. Deer are nevertheless important in transporting ticks and maintaining tick populations.

Other suggested modes of transmission

Person-to-Person
There is no evidence that Lyme disease is transmitted from person-to-person. For example, a person cannot get infected from touching, kissing or having sex with a person who has Lyme disease.

During Pregnancy & While Breastfeeding
Lyme disease acquired during pregnancy may lead to infection of the placenta and possible stillbirth, however, no negative effects on the fetus have been found when the mother receives appropriate antibiotic treatment. There are no reports of Lyme disease transmission from breast milk.

From Blood
Although no cases of Lyme disease have been linked to blood transfusion, scientists have found that the Lyme disease bacteria can live in blood that is stored for donation. Individuals being treated for Lyme disease with an antibiotic should not donate blood. Individuals who have completed antibiotic treatment for Lyme disease may be considered as potential blood donors.

From Pets
Although dogs and cats can get Lyme disease, there is no evidence that they spread the disease directly to their owners. However, pets can bring infected ticks into your home or yard. Consider protecting your pet, and possibly yourself, through the use of tick control products for animals.

Other Transmission
You will not get Lyme disease from eating venison or squirrel meat, but in keeping with general food safety principles meat should always be cooked thoroughly. Note that hunting and dressing deer or squirrels may bring you into close contact with infected ticks.

There is no credible evidence that Lyme disease can be transmitted through air, food, water, or from the bites of mosquitoes, flies, fleas, or lice.