Roughly 1 in 5 blacklegged ticks in the United States carry the bacterium that causes Lyme disease, but that national average hides enormous variation. Depending on where you live, the tick’s life stage, and the species involved, infection rates range from as low as 1% to as high as 50%.
Not All Ticks Carry Lyme
Only blacklegged ticks (also called deer ticks) transmit Lyme disease in the U.S. The most common species is Ixodes scapularis, found throughout the eastern half of the country. On the West Coast, the western blacklegged tick (Ixodes pacificus) is the carrier. Dog ticks, lone star ticks, and other common species do not transmit the Lyme bacterium, no matter how many of them you find on your skin.
So when people ask “how many ticks carry Lyme disease,” the answer starts with species. If you were bitten by something other than a blacklegged tick, Lyme is not a concern from that particular bite.
Infection Rates in the Northeast and Midwest
The Northeast and upper Midwest are the epicenter of Lyme disease in the U.S., and the numbers there are striking. A 2025 Dartmouth study published in Parasites & Vectors found that 50% of adult blacklegged ticks in the Northeast carry the Lyme bacterium. That’s a coin flip every time an adult tick latches on.
Nationally, molecular testing of more than 16,000 ticks found the Lyme bacterium in about 20% of adult Ixodes scapularis ticks, 11% of nymphs, and roughly 5% of larvae. Those numbers are averages across the entire country, though. In high-risk states like Connecticut, Massachusetts, Minnesota, and Wisconsin, local rates run considerably higher.
Since 1995, the geographic range of Lyme cases has expanded significantly, according to CDC surveillance maps comparing 1995, 2010, and 2023. Ticks carrying the bacterium are showing up in counties and states where they were previously rare, which means people in newly affected areas may not yet be accustomed to checking for ticks or recognizing early symptoms.
Rates Are Much Lower on the West Coast
California’s western blacklegged tick carries Lyme at far lower rates. Only about 1% to 2% of adult western blacklegged ticks are infected with the bacterium, compared to 20% or more for their eastern cousins. Nymphal ticks in California range from 2% to 15%, depending on the region.
This difference is partly ecological. Western blacklegged ticks in their nymphal stage tend to feed on a lizard species whose blood actually kills the Lyme bacterium, breaking the cycle of transmission. The result: Lyme disease exists on the West Coast but is far less common than in the Northeast.
Nymphs vs. Adults: Which Stage Is Riskier?
Adult ticks are more likely to be infected, but nymphal ticks cause most human Lyme cases. That sounds contradictory, and the explanation is simple: nymphs are tiny (about the size of a poppy seed) and much harder to spot. Adults are roughly the size of a sesame seed, engorged females even larger, so people tend to find and remove them faster.
Up to 25% of nymphal blacklegged ticks in the Northeast carry the Lyme bacterium. Their peak feeding season runs from late spring through summer, which is when the majority of Lyme infections occur. Because they’re so small, nymphs can feed for a full day or longer before you notice them, and that matters for transmission timing.
Attachment Time and Actual Transmission
Even if a tick is carrying the Lyme bacterium, it generally needs to be attached for more than 24 hours before transmission occurs. The bacterium lives in the tick’s gut and takes time to migrate to its salivary glands, where it can enter your bloodstream.
This creates a meaningful window for prevention. Finding and removing a tick within the first 24 hours dramatically reduces your risk, even if the tick was infected. Daily tick checks after spending time outdoors, especially during nymph season (May through July), are one of the most effective things you can do. Pay close attention to hidden areas: behind the ears, along the hairline, in the armpits, behind the knees, and around the waistband.
Some Ticks Carry More Than Just Lyme
Blacklegged ticks don’t just carry the Lyme bacterium. The same ticks can harbor other pathogens simultaneously. Nationally, about 4% of blacklegged ticks carry the bacterium that causes anaplasmosis, and 2% carry the parasite responsible for babesiosis. Nearly 1% of tested ticks were co-infected with both the Lyme bacterium and the anaplasmosis pathogen, making that the most common two-pathogen combination.
In the northeastern U.S., co-infection rates are considerably higher. Up to 28% of tested ticks in some northeastern areas carried multiple pathogens. This matters because co-infections can complicate symptoms and make diagnosis harder. A single tick bite can potentially deliver more than one disease at once, though triple infections (Lyme, anaplasmosis, and babesiosis together) were found in only 0.1% of ticks tested.
Putting the Numbers in Perspective
If you live in the Northeast or upper Midwest and spend time outdoors, roughly half the adult blacklegged ticks in your area may carry Lyme. If you’re in California, that number drops below 2% for adults. Nationally, the overall rate sits around 20% for adults and 11% for nymphs, but your actual risk depends heavily on your specific region.
The percentage of infected ticks is only one part of the equation. How quickly you find the tick, how long it’s been feeding, and whether you remove it properly all influence whether an infected tick actually transmits the disease. A tick that’s been on you for six hours, even one carrying the bacterium, is far less dangerous than one that’s been quietly feeding for two days in a spot you didn’t think to check.