Lyme disease is caused by infection with bacteria of the Borrelia genus, primarily Borrelia burgdorferi and sometimes Borrelia mayonii in North America. These bacteria are transmitted to humans through the bite of infected hard-bodied ticks of the genus Ixodes, such as the blacklegged tick (deer tick) in the United States. The ticks acquire the bacteria from feeding on infected small mammals and birds. The bacteria enter the skin through the tick bite and can spread throughout the body, leading to various symptoms. Transmission typically occurs if the tick is attached for more than 24 to 72 hours, with nymphal ticks being the most common transmitters due to their small size and likelihood of undetected attachment.