Lyme disease is caused by a bacteria called Borrelia burgdorferi. It is typically transmitted to humans through the bite of infected black-legged ticks (also known as deer ticks) that are typically found in wooded or grassy areas. The bacteria can be found in the tick’s saliva and can enter the human body when the tick feeds on a person’s blood. Lyme disease is most commonly found in the northeastern and north-central parts of the United States, as well as in parts of Europe and Asia.