Home/Lyme questions/What causes Lyme disease
Quick answerWhat causes Lyme disease?
The short version: bacteria carried by certain ticks. Here is which bacteria, which ticks, and how the germ gets from wildlife to people.
Quick answer
Lyme disease is caused by bacteria called Borrelia burgdorferi and, rarely, Borrelia mayonii. People get infected when a blacklegged (deer) tick carrying the bacteria bites them and stays attached long enough to pass it on. Ticks pick up the germ by feeding on infected small animals, mostly rodents.
- In the U.S., Lyme disease is caused by Borrelia burgdorferi and, rarely, Borrelia mayonii (CDC).
- It is spread by the bite of infected blacklegged ticks: the deer tick in the East and Midwest and the western blacklegged tick on the Pacific coast.
- Infected ticks generally need to be attached for more than 24 hours to spread the infection (CDC).
The Lyme disease pathogen
A pathogen is a germ that causes illness. For Lyme disease, it is a spiral-shaped bacterium in the Borrelia group. Borrelia burgdorferi causes nearly all U.S. cases. Borrelia mayonii is a related species that CDC says causes Lyme disease only rarely.
Because the cause is a bacterium, Lyme disease is treated with antibiotics. CDC says most cases can be treated successfully with a few weeks of them. See Lyme disease treatment.
How ticks pick up the bacteria
Ticks are not born infected. CDC explains that infected female ticks do not pass the bacteria to their eggs. Instead, young ticks (larvae and nymphs) pick it up when they feed on infected wild animals, usually rodents. The tick keeps the bacteria as it grows into its next stage, and can then pass it to a person or pet at its next blood meal.
Deer are part of the story, but not the source of the germ. Adult ticks feed on deer, which helps tick numbers grow, yet deer are not infected and do not infect ticks (CDC).
Nymphs cause many infections because they are tiny, about the size of a poppy seed, and are most active from April through July. Many people never notice the bite.
Which ticks spread Lyme disease?
| Tick | Spreads Lyme? | Where |
|---|---|---|
| Blacklegged tick (deer tick) | Yes | Northeast, mid-Atlantic, north-central states |
| Western blacklegged tick | Yes | Pacific coast |
| Lone star tick | No | Can spread other illnesses |
| American dog tick | No | Can spread other illnesses |
| Rocky Mountain wood tick, brown dog tick | No | Can spread other illnesses |
Infection rates in blacklegged ticks range from none to more than half, depending on the area and life stage. In most of the Southeast they are almost never infected (CDC). Read more in ticks that carry Lyme disease.
What does not cause Lyme disease
CDC says there is no credible evidence that Lyme disease spreads person to person through touching, kissing or sex, or through mosquitoes, fleas, lice, air, food or water. Pets can get Lyme disease but do not seem to pass it to people directly, though they can carry ticks into the home. See how Lyme disease is transmitted and do mosquitoes carry Lyme disease?
We are collecting real experiences about how they think they got Lyme disease from people who have lived it, shared with their permission. As stories come in, they will appear here, clearly marked as personal experience rather than medical advice.
Frequently asked questions
Is Lyme disease caused by a virus?
No. It is caused by bacteria, mainly Borrelia burgdorferi. That is why antibiotics work.
Do deer cause Lyme disease?
Not directly. Deer feed adult ticks and help tick populations grow, but CDC says deer are not infected and do not infect ticks.
Can any tick bite cause Lyme disease?
No. In the U.S. only blacklegged ticks and western blacklegged ticks spread it. Lone star ticks and American dog ticks do not (CDC).