Roughly 20 to 50 percent of deer ticks carry the bacterium that causes Lyme disease, depending on the tick’s life stage, geographic location, and local ecology. That’s a wide range because infection rates vary dramatically from one region to the next. In parts of the Northeast, half of all adult deer ticks are infected. On the West Coast, the number drops to as low as 1 to 2 percent.
National and Regional Infection Rates
There is no single national average for deer tick infection because the rates shift so much by region. The most detailed picture comes from a Dartmouth-led meta-analysis published in Parasites & Vectors, which pooled data from 1989 to 2021 across Connecticut, New York, New Hampshire, Vermont, and Maine. That analysis found 50 percent of adult blacklegged ticks in the Northeast carry the Lyme bacterium, while up to 25 percent of nymphs (the younger, smaller life stage) are infected.
Those numbers represent some of the highest-risk areas in the country. In emerging Lyme regions, like parts of eastern Canada, overall infection prevalence among collected ticks runs around 32 percent when adults and nymphs are combined. In contrast, the western black-legged tick found in California carries the bacterium at much lower rates: typically 1 to 2 percent of adults and 2 to 15 percent of nymphs in northern California, according to the University of California’s Integrated Pest Management program. If you live on the West Coast, your odds of encountering an infected tick are significantly lower than if you live in New England or the mid-Atlantic.
Adults vs. Nymphs: Why Life Stage Matters
Adult deer ticks are consistently more likely to be infected than nymphs. Adults have fed twice in their lifetime (once as a larva, once as a nymph), giving them two opportunities to pick up the bacterium from an infected host like a white-footed mouse. Nymphs have fed only once. Studies consistently show adult infection rates roughly 10 to 15 percentage points higher than nymphal rates in the same area.
Despite being less likely to carry the bacterium, nymphs cause the majority of human Lyme disease cases. They’re active in late spring and summer, when people spend the most time outdoors, and they’re tiny, about the size of a poppy seed. Most people who develop Lyme never noticed the tick that bit them, and that tick was almost certainly a nymph. Adults are active in fall and early spring, are much easier to spot, and are more likely to be found and removed before they can transmit the infection.
How Location and Habitat Shift the Numbers
Even within a single county, infection rates can vary block by block. Ticks collected at the edge where residential yards meet woodland, the so-called interface zone, are far more likely to be infected than ticks found deep in the forest or in open residential areas. One study in an emerging Lyme region found ticks at these interface zones were roughly 15 to 24 times more likely to carry the bacterium compared to ticks from purely residential or purely wooded zones. This makes sense: the interface is where tick-carrying wildlife like deer and mice overlap most with humans.
Climate plays a role too. Ticks are cold-blooded, so temperature and humidity directly affect their survival, activity levels, and how aggressively they seek hosts. Warmer, more humid conditions generally increase tick activity and can extend the seasons when ticks are questing for a blood meal. In the eastern United States, the expansion of wooded areas into previously cleared land, combined with more homes being built at the edges of forests, has increased contact between ticks and people over the past several decades.
An Infected Tick Doesn’t Guarantee Infection
Even if a tick is carrying the Lyme bacterium, transmission to a human isn’t instant. According to the CDC, the tick generally needs to be attached for more than 24 hours before the bacterium can make its way into your bloodstream. The Lyme-causing spirochete lives in the tick’s gut and needs time to migrate to the salivary glands before it can enter the bite wound. This is why prompt tick checks after spending time outdoors are so effective at preventing Lyme disease. If you find and remove a tick within 24 hours of attachment, your risk drops substantially.
The tick also needs to be actively feeding, not just crawling on your skin. A tick that hasn’t bitten you poses no transmission risk regardless of what it’s carrying.
Ticks Often Carry More Than Just Lyme
The Lyme bacterium isn’t the only pathogen deer ticks transmit. A study that tested ticks from Indiana, Maine, Pennsylvania, and Wisconsin found that among ticks carrying at least one pathogen, 45 percent were co-infected with two or more disease-causing organisms. The most common co-infections involved the agents responsible for anaplasmosis and babesiosis alongside the Lyme bacterium. About 8 percent of infected ticks harbored three different pathogens, and one tick collected in Wisconsin carried four.
This matters because co-infections can complicate diagnosis and treatment. Someone bitten by a tick carrying both the Lyme bacterium and the babesiosis parasite, for example, may have symptoms that don’t fit a straightforward Lyme presentation. If you develop symptoms after a tick bite and don’t improve as expected, co-infection is one reason your healthcare provider may broaden testing beyond Lyme alone.
Putting the Numbers in Perspective
The percentage of ticks carrying Lyme varies enough that a single number is misleading. A practical way to think about your personal risk: if you live in or visit the Northeast or upper Midwest, roughly one in every two to four adult deer ticks you encounter could be carrying the bacterium. For nymphs in those regions, it’s closer to one in four or five. On the West Coast, your odds are much lower, closer to one in 50 to 100 for adults.
Regardless of the local infection rate, the same prevention strategies apply. Wearing treated clothing, using repellent, and doing a thorough tick check within a few hours of coming indoors are the most reliable ways to prevent Lyme. The 24-hour transmission window gives you a real buffer, but only if you actually look for ticks. A tick you don’t find is the one that transmits disease.