Lyme disease is caused by a corkscrew-shaped bacterium called Borrelia burgdorferi, transmitted to humans through the bite of infected blacklegged ticks. It is the most common vector-borne disease in North America, with an estimated 476,000 people diagnosed and treated for it each year in the United States alone. Over 89,000 cases were reported to the CDC in 2023.
The Bacterium Behind Lyme Disease
Borrelia burgdorferi is a spirochete, a type of bacterium with a distinctive spiral shape that allows it to bore through tissue. In North America, B. burgdorferi is the primary species responsible for Lyme disease, though a second species called B. mayonii can also cause it in the upper Midwest.
In Europe and Asia, the picture is different. Two additional species cause Lyme disease there: B. afzelii, which circulates mainly through small rodents, and B. garinii, maintained primarily by birds. Of the two, B. afzelii tends to be more prevalent. Altogether, the broader Borrelia complex contains over 20 known species, but only a handful actually cause disease in humans.
How Ticks Transmit the Bacteria
The blacklegged tick (sometimes called the deer tick) is the carrier in the eastern United States, where it is widely distributed. Along the Pacific coast, particularly in northern California, the western blacklegged tick fills the same role. No other tick species in the U.S. is a significant transmitter of Lyme disease. A recent experimental study found that the Asian longhorned tick, a newer arrival in the country, is not likely to contribute to the spread of the bacteria.
The transmission process begins inside the tick’s gut. Borrelia bacteria live in the tick’s midgut, essentially dormant while the tick waits for its next meal. When the tick latches onto a host and begins feeding, the incoming blood triggers changes in the bacteria’s outer surface proteins. Over the first 24 to 72 hours of feeding, the bacteria shift their protein coat, switching off one surface protein and activating another. This molecular costume change is what allows them to migrate from the midgut, through the tick’s body cavity, and into its salivary glands, where they can be injected into the host’s skin.
This migration takes time, which is why tick attachment duration matters so much. In most cases, an infected tick must be attached for more than 24 hours before it can transmit the bacteria. The critical window for bacterial migration from midgut to salivary glands falls between 48 and 72 hours of feeding. Removing a tick promptly, ideally within 24 hours, can prevent transmission entirely.
How the Bacteria Spread Through Your Body
Once injected into the skin, B. burgdorferi doesn’t just sit at the bite site. The bacterium uses specialized surface proteins that bind to components of your connective tissue, the structural framework between your cells. These proteins act like molecular grappling hooks, allowing the spirochete to pull itself through dense tissue that would otherwise trap it. Researchers have found that when these adhesion proteins are disabled, the bacteria disseminate far less effectively to organs distant from the original bite.
This ability to spread is what makes Lyme disease a systemic illness rather than a localized skin infection. Without treatment, the bacteria can reach joints, the heart, and the nervous system over a period of days to weeks.
Which Tick Life Stage Is Most Dangerous
Ticks go through three active life stages: larva, nymph, and adult. They typically pick up the Borrelia bacteria during their larval or nymphal stages by feeding on infected animals, especially white-footed mice. They then transmit those pathogens to other animals or humans during later feedings as nymphs or adults.
Nymphs are widely considered the most dangerous stage for human infection. They’re roughly the size of a poppy seed, small enough to go unnoticed on the skin for the 24-plus hours needed to transmit the bacteria. Adult ticks are larger and more likely to be spotted and removed before transmission occurs. Larvae can carry the bacteria after feeding on an infected animal, but they rarely bite humans.
Where Risk Is Highest
Your environment determines your exposure. Blacklegged ticks thrive in deciduous forests, particularly those dominated by oak and maple trees, with dry to moderately moist conditions and sandy or loamy soils. Tick densities are highest in oak-dominated forests, followed by maple forests. Conifer forests with minimal leaf litter support far fewer ticks, and grasslands are largely tick-free.
The leaf litter layer on the forest floor is critical. It provides the humidity ticks need to survive and shelters the small mammals they feed on. If you live near or spend time in wooded areas with thick leaf litter in the northeastern, mid-Atlantic, or upper midwestern United States, or along the northern California coast, your risk is highest. Peak transmission season runs from late spring through summer, when nymphs are most active.
Co-infections From the Same Tick Bite
The same ticks that carry Borrelia can simultaneously carry other pathogens. In the U.S., the most common co-infection alongside Lyme disease is babesiosis, caused by a parasite that infects red blood cells. Anaplasmosis, a bacterial infection, is the second most common. Depending on geography, between 4% and 45% of Lyme disease patients in endemic areas also test positive for babesiosis or anaplasmosis.
Co-infections tend to produce more severe and prolonged symptoms than Lyme disease alone. People infected with both Borrelia and the babesiosis parasite experience more fatigue, headaches, sweats, chills, and nausea, and their illness lasts longer. Anaplasmosis co-infection may also lead to a more severe form of Lyme disease with a longer recovery. In Europe, tick-borne encephalitis virus is a more common co-infection than babesiosis, reflecting the different pathogen landscape there.
A single tick bite, in other words, can deliver more than one infection at once. This is one reason Lyme disease symptoms sometimes don’t follow the textbook pattern, and why persistent or unusual symptoms after a tick bite warrant thorough evaluation.