The percentage of ticks carrying Lyme disease depends heavily on where you live and the tick’s life stage. In the northeastern United States, about 50% of adult blacklegged ticks carry the Lyme-causing bacteria, while on the West Coast, that number drops to just 1 to 2%. These are dramatically different odds, and understanding them can help you assess your actual risk after a tick bite.
Not All Ticks Carry Lyme
Only blacklegged ticks (also called deer ticks) transmit Lyme disease in the U.S. The species found in the East and Midwest is Ixodes scapularis, while the West Coast has a related species, Ixodes pacificus. Dog ticks, lone star ticks, and other common species do not transmit the Lyme bacteria at all, no matter how long they’re attached. So the first question after finding a tick on your body is which kind it is.
Infection Rates in the Northeast and Midwest
A 2025 Dartmouth study published in Parasites & Vectors analyzed data spanning 1989 to 2021 across Connecticut, New York, New Hampshire, Vermont, and Maine. The findings: 50% of adult blacklegged ticks in these states carry the Lyme bacterium, and up to 25% of nymphal ticks (the younger, smaller stage) are infected. These are the highest regional rates in the country, which tracks with the fact that the Northeast and upper Midwest account for the vast majority of Lyme disease cases each year.
Infection Rates on the West Coast
The western blacklegged tick carries Lyme at far lower rates. In northern California, where most West Coast Lyme cases originate, only about 1 to 2% of adult ticks are infected. Nymphal ticks in the same region carry the bacteria at rates between 2 and 15%, according to the University of California’s agriculture program. The reasons for this gap likely involve differences in the animal hosts these ticks feed on. Western ticks more commonly bite lizards, which don’t sustain the Lyme bacterium the way white-footed mice in the East do.
Why the Tick’s Life Stage Matters
Ticks go through three feeding stages: larva, nymph, and adult. Larvae are essentially born clean and rarely carry Lyme. Nymphs pick up the bacteria from infected animals during their first blood meal and carry it forward, so about 25% of nymphs in the Northeast are infected. Adults have had two chances to feed on infected hosts, which is why their infection rate climbs to around 50% in high-risk areas.
Here’s the catch: nymphs are actually responsible for most human Lyme infections. They’re roughly the size of a poppy seed, making them extremely easy to miss on your skin. Adults are larger and more likely to be noticed and removed before they can transmit the bacteria. So a lower infection rate in nymphs doesn’t necessarily mean lower risk to you.
Attachment Time and Transmission Risk
Even if a tick is carrying the Lyme bacterium, it doesn’t transmit it instantly. The bacteria live in the tick’s gut and need time to migrate to its salivary glands before entering your bloodstream. Experimental studies on infected nymphal ticks found zero confirmed transmissions when ticks were attached for only 24 hours, despite testing on nearly 90 animal hosts across multiple studies. By 48 hours of attachment, the probability of transmission rises to about 10%. By 72 hours, it reaches roughly 70%.
This timeline is why daily tick checks are so effective. If you find and remove a tick within a day, your chances of contracting Lyme are extremely low, even if the tick was infected.
Seasonal Patterns in Infection Rates
Tick activity and tick infection rates don’t peak at the same time. Research in New York found that tick density is highest during summer (June through August), but the proportion of ticks that are actually infected is significantly higher in spring (April and May) and fall (September through November). This means a tick bite in April may carry greater odds of Lyme exposure than one in July, even though you’ll encounter more ticks in midsummer.
Ticks That Carry More Than One Pathogen
Blacklegged ticks don’t just carry Lyme. They can also harbor the organisms that cause anaplasmosis, babesiosis, and other infections. A review of 61 published studies found that 2 to 5% of nymphal blacklegged ticks carried more than one pathogen simultaneously. For adult ticks, co-infection rates ranged widely, from 1 to 28% depending on the region and study. Co-infections can complicate symptoms and make diagnosis harder, since the signs may overlap or not fit the typical Lyme pattern.
Putting Your Risk in Perspective
Your actual risk after a tick bite is the product of several factors: whether it was a blacklegged tick, whether that tick was infected, how long it was attached, and how quickly you received treatment if needed. In a worst-case scenario (an adult blacklegged tick in Connecticut attached for three days), the math works out to roughly a 35% chance of transmission. In a best-case scenario (a tick found and removed within hours, or a species that doesn’t carry Lyme at all), the risk is essentially zero.
Geography is the single biggest variable. If you live in or visited the Northeast, upper Midwest, or mid-Atlantic states, take every blacklegged tick bite seriously. If you’re on the West Coast, the odds are much more in your favor, though Lyme cases do still occur there. Wherever you are, removing ticks promptly remains the most reliable way to prevent infection.