Roughly 20 to 50% of blacklegged ticks carry the bacterium that causes Lyme disease, depending on the tick’s life stage and where in the country you are. That’s a wide range because infection rates vary dramatically by region. In parts of the Northeast, half of all adult blacklegged ticks are infected. On the West Coast, the number drops to as low as 1 to 2%.
Not All Ticks Carry Lyme
Only blacklegged ticks (also called deer ticks) transmit Lyme disease in the United States. The eastern species lives throughout the eastern half of the country, while the western blacklegged tick is found along the Pacific coast. Dog ticks, lone star ticks, and other common species do not transmit the Lyme bacterium, even though they can carry other infections. So if you find a tick on your body, the species matters as much as whether it was infected.
Infection Rates in the Northeast and Upper Midwest
A large meta-analysis published in Parasites & Vectors pulled together data from 1989 to 2021 across Connecticut, New York, New Hampshire, Vermont, and Maine. The results: about 50% of adult blacklegged ticks in the Northeast carry the Lyme bacterium. For nymphs, the younger and smaller life stage, up to 25% are infected.
These are the highest rates in the country. The Northeast and Upper Midwest (Minnesota, Wisconsin) are where most Lyme disease cases originate, and the tick population density in these regions is also far greater than elsewhere. So it’s a double hit: more ticks, and a higher proportion of those ticks are carrying the bacterium.
Why Nymphs Are More Dangerous Than Adults
Adult blacklegged ticks are easier to spot. They’re roughly the size of a sesame seed before feeding. Nymphs, by contrast, are about the size of a poppy seed, small enough to hide in a skin fold, behind an ear, or along a hairline without being noticed. Even though nymphs have a lower infection rate (up to 25% versus 50% for adults in the Northeast), they cause the majority of Lyme disease cases precisely because people don’t find them in time.
Timing matters here. An infected tick generally needs to be attached for more than 24 hours before it transmits the bacterium. That’s because the Lyme-causing organism lives in the tick’s gut and needs time to migrate to the salivary glands before entering your bloodstream. Finding and removing a tick within the first day significantly lowers your risk. A tiny nymph that goes undetected for two or three days is far more likely to transmit the infection than a large adult you notice and pull off within hours.
Much Lower Rates on the West Coast
California and other Pacific coast states have a different picture entirely. The western blacklegged tick is less efficient at transmitting Lyme, and infection rates are much lower. In northern California, where Lyme does occur, only about 1 to 2% of adult ticks and 2 to 15% of nymphs carry the bacterium. The variation within that nymph range depends on the specific habitat. Ticks collected from dense woodland with high rodent populations tend to have higher infection rates than those from drier, more open landscapes.
This is one reason Lyme disease is far less common on the West Coast. California reports a fraction of the cases seen in states like Connecticut or Pennsylvania, even though blacklegged ticks are present.
What These Numbers Mean for You
Finding a blacklegged tick on your skin does not mean you’ll get Lyme disease. Even in the Northeast, where half of adult ticks are infected, the bacterium still requires prolonged attachment to transfer. Your actual risk after a tick bite depends on three factors: whether the tick was a blacklegged tick, whether it was infected, and how long it was attached.
If you remove a tick that’s still flat and hasn’t visibly engorged with blood, it likely hasn’t been feeding long enough to transmit the bacterium. An engorged tick that’s swollen and darker in color has been attached longer, which increases the risk. Save the tick in a sealed bag or photograph it so you can identify the species later.
The CDC tracks where blacklegged ticks and the Lyme bacterium have been found across the country, and the range of these ticks has been expanding southward and inland over the past two decades. Areas that historically had few or no blacklegged ticks, including parts of the mid-Atlantic and Ohio Valley, are now reporting established populations. That means infection rate data from your specific county or state gives you a better picture than national averages.
Checking the Numbers for Your Area
Local and state health departments often conduct tick surveillance programs that test ticks for the Lyme bacterium and publish the results. Some states also run tick identification services where you can mail in a tick you’ve removed and get it tested. University extension programs, particularly in the Northeast, maintain updated maps showing infection rates by county.
The overall pattern is consistent: the closer you are to the Northeast or Upper Midwest epicenter, the higher the proportion of infected ticks. In those regions, treating any blacklegged tick bite with appropriate caution is reasonable. In lower-risk areas like the Southeast or West Coast, the odds are substantially lower, but not zero.