The percentage of ticks carrying Lyme disease bacteria ranges from zero to over 50%, depending on where you live, the species of tick, and whether the tick is a juvenile or an adult. Nationally, there is no single number that applies everywhere. In the Northeast, about half of adult blacklegged ticks carry the Lyme bacterium, while in northern California, only 1 to 2% of adult ticks of the relevant species are infected.
Understanding these numbers matters, but they’re only part of the picture. The species of tick, its life stage, how long it’s been attached, and your geographic region all shape your actual risk of infection after a bite.
Only Certain Tick Species Carry Lyme
Not every tick can transmit Lyme disease. In the eastern and central United States, the blacklegged tick (sometimes called the deer tick) is the only species that spreads the Lyme bacterium. On the West Coast, a close relative called the western blacklegged tick fills that role. Common species like the American dog tick, the lone star tick, and the brown dog tick do not transmit Lyme disease at all, regardless of where they’re found.
So if you’ve been bitten by a tick, the first question isn’t “what percentage are infected?” but “what kind of tick bit me?” If it’s not a blacklegged tick, your Lyme risk from that bite is essentially zero.
Infection Rates in the Northeast
The northeastern United States has the highest tick infection rates in the country. A 2025 Dartmouth study that pooled data from Connecticut, New York, New Hampshire, Vermont, and Maine spanning 1989 to 2021 found that 50% of adult blacklegged ticks carry the Lyme bacterium. Among younger ticks, called nymphs, up to 25% were infected.
Those are striking numbers. If you find an adult blacklegged tick on your body in New England or the Mid-Atlantic, there’s roughly a coin-flip chance it’s carrying the pathogen. CDC surveillance in nine national parks across the Northeast and Mid-Atlantic confirmed that the Lyme bacterium was present at every site where blacklegged ticks were collected, though density varied widely from park to park.
Infection Rates on the West Coast
The picture in California and the Pacific Northwest is very different. In northern California, where Lyme disease does occur, only about 1 to 2% of adult western blacklegged ticks are infected. Nymphs in the same areas carry the bacterium at rates of 2 to 15%, according to the University of California’s pest management program.
This means your risk from a single tick bite in California is dramatically lower than from a bite in Connecticut or Maine. It also explains why Lyme disease case counts are concentrated so heavily in the Northeast and upper Midwest.
Nymphs vs. Adults: Which Stage Is Riskier?
Adult blacklegged ticks are more likely to be infected than nymphs. Adults in the Northeast carry the bacterium about 50% of the time, compared to roughly 25% for nymphs. Data from Ohio showed a similar pattern: 21.4% of adults were infected versus 13.5% of nymphs.
Despite their lower infection rate, nymphs actually cause the majority of Lyme disease cases in humans. The reason is practical: nymphs are tiny, roughly the size of a poppy seed, and much harder to notice on your skin. Adults are closer to the size of a sesame seed and more likely to be found and removed before they’ve fed long enough to transmit the bacterium. Most Lyme transmission happens in late spring and summer, when nymphs are most active.
Attachment Time Changes Everything
Even if a tick is infected, it generally needs to be attached for more than 24 hours before the Lyme bacterium can be transmitted to you. The CDC notes that removing a tick within 24 hours greatly reduces your chances of getting Lyme disease. This is because the bacterium lives in the tick’s gut, and it takes time for it to migrate to the tick’s salivary glands and enter your bloodstream during feeding.
This means the raw infection percentage of ticks in your area is less important than how quickly you find and remove an attached tick. A daily tick check after spending time outdoors is one of the most effective things you can do to prevent Lyme disease, even in areas where half of all adult ticks are infected.
Why Infection Rates Vary So Much by Location
The percentage of infected ticks in a given area is largely driven by the local population of white-footed mice. These small rodents are the primary host responsible for infecting ticks with the Lyme bacterium. Larval ticks feed on many types of animals, but white-footed mice are the most efficient source of infection. When mouse populations are high, a greater proportion of ticks successfully feed on them, and more ticks pick up the bacterium in the process.
Research from the Cary Institute of Ecosystem Studies found a statistically significant relationship between mouse density and the proportion of ticks that find a mouse host. In years and locations with more mice, more ticks become infected as larvae and then carry the bacterium into their nymph and adult stages, when they’re most likely to bite humans. This is one reason infection rates can differ sharply between two wooded areas just miles apart.
Ticks Can Carry More Than One Pathogen
Blacklegged ticks don’t just carry Lyme disease. The CDC tracks seven different pathogens in these ticks, including the agents that cause anaplasmosis, babesiosis, and Powassan virus disease. A five-year surveillance study in Maryland found that co-infections in individual ticks were rare, with most infected ticks carrying just one pathogen. The most common co-infection pairing, Lyme bacteria plus the anaplasmosis agent, showed up in about 2.7% of nymphs that had fed on white-footed mice. Triple infections carrying all three common pathogens were detected only once across hundreds of ticks.
While co-infections are uncommon in any single tick, they do occur, and being infected with two tick-borne pathogens simultaneously can complicate symptoms and treatment. This is another reason prompt tick removal matters so much.
Putting the Numbers in Perspective
If you live in the Northeast or upper Midwest, roughly 1 in 4 nymphal ticks and 1 in 2 adult ticks you encounter could be carrying Lyme bacteria. On the West Coast, those numbers drop to somewhere between 1 in 50 and 1 in 7, depending on life stage and specific location. In areas where blacklegged ticks haven’t been established, the risk is essentially nonexistent.
The percentage of infected ticks tells you about background risk in your environment, but your personal risk from any single bite depends on three things: whether the tick was a blacklegged tick, whether it was infected, and whether it was attached long enough to transmit. Removing ticks promptly, checking your body after outdoor activity, and knowing what blacklegged ticks look like at different life stages all reduce your risk far more than any regional statistic might suggest.