Roughly 15 to 20 percent of nymphal black-legged ticks in the northeastern United States carry the bacterium that causes Lyme disease. That number shifts depending on where you live, which species of tick bit you, and whether the tick is a nymph or an adult. Not every tick is a Lyme carrier, and not every bite from an infected tick leads to illness.
Infection Rates in the Northeast and Midwest
The black-legged tick (sometimes called the deer tick) is the primary carrier of Lyme disease in the eastern half of the country. In residential areas of Connecticut, Maryland, and New York, studies have found nymphal infection rates between 15 and 23 percent, with an overall average around 18 percent. That means roughly one in five to one in six nymphs collected from yards and surrounding vegetation tested positive for the Lyme bacterium.
These rates fluctuate year to year and property to property. In one year of sampling, Connecticut nymphs tested positive at about 16 percent, while New York nymphs hit 23 percent. The following year, Connecticut’s rate dropped to around 9 percent while New York stayed near 23 percent. Local wildlife populations, habitat type, and climate all influence how many ticks pick up the bacterium during their feeding cycle.
How Rates Differ on the West Coast
The western black-legged tick, found along the Pacific Coast, carries Lyme disease bacteria at notably lower rates. In southern Oregon, nymphs showed infection rates of roughly 1.6 percent, while adults came in at about 0.3 percent. A Washington state study found around 4 percent of adult ticks and 7 percent of nymphs were infected. Studies across California have consistently found the same pattern: infection rates in western ticks are a fraction of what you see in the Northeast.
This is one reason Lyme disease is far more common in the northeastern and upper midwestern states. The ticks there are simply more likely to be carrying the bacterium, and they tend to be present in higher numbers in residential landscapes.
Nymphs vs. Adults
Tick life stage matters in two ways: how likely the tick is to be infected, and how likely it is to bite you unnoticed. Nymphs are tiny, about the size of a poppy seed, and they’re responsible for the majority of Lyme disease cases because people don’t feel or see them before they’ve been attached for hours.
Adult ticks tend to carry the bacterium at higher rates than nymphs, since they’ve had an additional blood meal and another chance to pick up the infection. European data on a related tick species shows adults testing positive at around 35 percent compared to 20 percent for nymphs. In the U.S. Northeast, adult infection rates typically run higher than the 15 to 20 percent seen in nymphs, though the exact figure varies by location. Despite being more frequently infected, adult ticks are larger, easier to spot, and more often removed before they can transmit the bacterium.
Carrying Lyme Isn’t the Whole Story
An infected tick on your skin doesn’t automatically mean you’ll get Lyme disease. The bacterium lives in the tick’s gut and needs time to migrate to its salivary glands before it can enter your bloodstream. The CDC states that an infected tick generally needs to be attached for more than 24 hours before transmission occurs. Removing a tick within that window greatly reduces your risk.
Even after a bite from a confirmed positive tick, the odds are in your favor. A large study tracking people after tick bites found the overall risk of developing Lyme disease from any single bite was about 2.6 percent. When the tick tested positive for the bacterium, that risk rose to about 6.7 percent. The worst-case scenario, a bite from a positive tick that had been feeding long enough to become visibly engorged, carried a risk of roughly 14 percent, or about one case per seven such bites. Attachment duration is a major factor: a positive tick removed within 12 hours posed about a 5 percent risk, while one left attached for four days pushed the risk closer to 13 or 14 percent.
Ticks Often Carry More Than One Pathogen
Black-legged ticks don’t just carry Lyme. They can harbor the agents that cause anaplasmosis, babesiosis, and other infections simultaneously. In a study of nearly 400 adult ticks collected across Indiana, Maine, Pennsylvania, and Wisconsin, 45 percent of infected ticks carried two or more pathogens at once. About 37 percent harbored two agents and 8 percent harbored three. The most common combination was the Lyme bacterium paired with another microbe.
This matters because co-infections can complicate symptoms and make diagnosis harder. If you develop a fever, body aches, or fatigue after a tick bite, especially in a high-prevalence area, the possibility of more than one infection is worth keeping in mind.
When Risk Is Highest
Tick activity peaks during the warmer months. The primary transmission season runs from April through November, with the highest risk concentrated in June through August. This lines up with nymphal activity, since nymphs are most active in late spring and early summer, precisely when people are spending the most time outdoors in shorts and sandals.
Risk drops significantly in winter in most of the country, though adult ticks can remain active on mild days even into late fall. The combination of high nymphal activity, warm weather drawing people outside, and the near-invisibility of nymphal ticks is what makes early summer the peak period for new Lyme infections each year.
Practical Takeaways by Region
- Northeast and upper Midwest: Roughly 15 to 25 percent of nymphal black-legged ticks carry the Lyme bacterium. Adult rates run even higher. This is the epicenter of Lyme disease in the U.S.
- West Coast: Western black-legged ticks carry the bacterium at rates typically below 5 percent for nymphs and often under 1 percent for adults. Lyme disease occurs here but is far less common.
- South and Southeast: The lone star tick is abundant in these regions but does not transmit Lyme disease. Black-legged ticks are present in some southern states, but infection rates and Lyme cases are much lower than in the Northeast.
The single most effective thing you can do is check your body thoroughly after spending time in grassy or wooded areas and remove any attached tick promptly. A tick found and pulled within 24 hours poses minimal risk of transmitting Lyme disease, regardless of whether it was carrying the bacterium.