Lyme disease comes from a bacterium that lives in certain ticks

Lyme disease is caused by a spiral-shaped bacterium called Borrelia burgdorferi. You cannot catch it from another person, from food, or from the air. You get it only when an infected tick bites you and stays attached long enough to pass the bacterium into your bloodstream. The tick itself gets infected when it feeds on an animal—usually a deer or small rodent—that already carries the bacterium.

The bacterium does not live in all ticks. It lives in specific species: the black-legged tick (also called the deer tick) in the eastern and central United States, and the western black-legged tick on the Pacific Coast. A tick can carry the bacterium for its entire life without getting sick, which is why infected ticks keep spreading it year after year.

Key Takeaways

  • Lyme disease is caused by the bacterium Borrelia burgdorferi, which lives inside certain tick species and is passed to humans only through tick bites.
  • The black-legged tick must remain attached to your skin for at least 24 to 48 hours to transmit the bacterium, which is why prompt removal matters.
  • Ticks pick up the bacterium by feeding on infected animals like deer and rodents, then carry it for life and pass it to the next host they bite.
  • Lyme disease risk is highest in the Northeast, Mid-Atlantic, and upper Midwest regions where infected ticks are most common.

How long a tick must stay attached to transmit the disease

The tick does not transmit the bacterium immediately. Research shows that an infected tick usually needs to stay attached for at least 24 to 48 hours before enough bacteria move from the tick's gut into your bloodstream. Some studies suggest transmission can happen faster—within 16 hours—but the longer the tick feeds, the higher your risk.

This timing matters because it means a tick you find and remove within a few hours is far less likely to have transmitted the disease. The bacterium lives in the tick's midgut, and it takes time for it to migrate to the tick's salivary glands, where it can enter your body. Removing the tick promptly is one of the few ways you can reduce your risk once a bite has happened.

Where infected ticks live and when they bite

Black-legged ticks live in wooded areas, tall grass, and leaf litter across much of the United States. They are most active in spring and early summer, though they can bite in fall and even on warm winter days. The ticks are tiny—nymphs (young ticks) are about the size of a poppy seed, and adults are only slightly larger—which is why people often do not notice them.

Lyme disease is not evenly distributed across the country. The vast majority of cases occur in the Northeast, Mid-Atlantic, and upper Midwest, where black-legged ticks are common and many of them carry the bacterium. In some areas of the Northeast, 20 to 40 percent of black-legged ticks carry Borrelia burgdorferi. In other regions, the percentage is much lower or the ticks are absent entirely.

How ticks become infected in the first place

Ticks are born without the bacterium. They become infected when they feed on an animal that already carries Borrelia burgdorferi. Deer are the most common source, but the bacterium also lives in rodents, birds, and other small mammals. Once a tick feeds on an infected animal, it carries the bacterium for the rest of its life—which can be several years.

This is why Lyme disease persists in nature. The bacterium cycles between ticks and animals in what scientists call an enzootic cycle. Infected ticks bite animals, the animals become infected, new ticks feed on those animals and become infected, and the cycle continues. Humans are accidental hosts—we are not part of the natural cycle, but we can be bitten by an infected tick that was meant to feed on a deer or mouse.

Why the bacterium causes symptoms weeks or months later

After the bacterium enters your body, it does not cause symptoms right away. It multiplies slowly and spreads through your tissues and bloodstream over days and weeks. The first sign is often the rash called erythema migrans, which typically appears 3 to 30 days after the tick bite. Some people never develop a rash, or the rash is small and easy to miss.

If the infection is not treated, the bacterium can spread to the joints, heart, and nervous system, causing symptoms that may not appear until weeks or months later. This delay is one reason Lyme disease can be hard to diagnose—people often do not remember the tick bite by the time they develop joint pain or neurological symptoms, and they may not connect the two events.

Other bacteria that ticks can transmit

Black-legged ticks can carry more than one disease-causing bacterium at the same time. A single tick bite can transmit Lyme disease, anaplasmosis (caused by Anaplasma phagocytophilum), and babesiosis (caused by Babesia microti) all at once. Babesiosis is a parasitic infection, not a bacterial one, but it spreads the same way—through an infected tick bite.

Rocky Mountain spotted fever, caused by Rickettsia rickettsii, is transmitted by a different tick species (the American dog tick and Rocky Mountain wood tick) and is more common in the South and West. Powassan virus is a newer concern in the Northeast and Midwest, also spread by black-legged ticks. The presence of multiple pathogens in the same tick population means that tick prevention is important even in areas where Lyme disease is less common.

Frequently Asked Questions

Can you get Lyme disease from a tick that is not attached to your skin?

No. The bacterium lives inside the tick and is transmitted through the tick's saliva during a blood meal. A tick crawling on your skin or clothing cannot transmit the disease. The tick must bite and remain attached for at least 24 to 48 hours.

If a tick bites me, will I definitely get Lyme disease?

No. Not all ticks carry the bacterium, and even an infected tick does not always transmit it. Your risk depends on the tick species, whether it is infected, how long it was attached, and your geographic location. In areas where Lyme disease is rare, your risk is very low.

Can you catch Lyme disease from someone who has it?

No. Lyme disease is not contagious from person to person. You cannot catch it through contact, saliva, blood, or any other direct exposure to an infected person. The only way to get Lyme disease is through a tick bite.

Why do some people get Lyme disease and others do not after a tick bite?

Several factors matter: whether the tick was actually infected, how long it was attached, where on your body it bit you, and possibly differences in how individual immune systems respond. A tick removed within 24 hours is much less likely to transmit the bacterium than one left attached for days.

Does removing a tick right away prevent Lyme disease?

Removing a tick within 24 hours significantly reduces your risk, but it does not may provide prevention. The bacterium can sometimes transmit within 16 hours, though this is less common. Prompt removal is still the best action you can take once you find a tick.