Lyme disease is caused by bacteria called Borrelia burgdorferi (B burgdorferi). Blacklegged ticks, also called deer ticks, can carry these bacteria. Not all species of ticks can carry these bacteria. Immature deer ticks, called nymphs, are the most common vectors for Lyme disease. They are about the size of a pinhead. Nymphs pick up bacteria when they feed on small rodents, such as mice, infected with B burgdorferi. You can get Lyme disease only if you are bitten by an infected tick.