In the northeastern United States, roughly 50% of adult deer ticks carry the bacterium that causes Lyme disease. That number drops to about 25% for nymphs, the younger, smaller life stage. These rates vary significantly by region, with some areas of the country seeing much lower infection rates in tick populations.
Infection Rates by Life Stage
Not all deer ticks are equally likely to be carrying Lyme. Adult blacklegged ticks, which are active primarily in fall and early spring, have the highest infection rates. In the Northeast, about half of them harbor the Lyme bacterium. Nymphs, which are active in late spring and summer, carry it at roughly half that rate, with up to 25% infected.
Despite the lower infection rate, nymphs actually cause the majority of Lyme disease cases in humans. They’re roughly the size of a poppy seed, making them far harder to spot on your skin. Adults are closer to the size of a sesame seed and much easier to find during a body check. Because nymphs go unnoticed longer, they’re more likely to stay attached long enough to transmit the bacterium.
Geography Makes a Big Difference
The 50% figure applies to high-risk areas in the Northeast and upper Midwest. If you live in the Southeast, the Pacific coast, or other parts of the country, infection rates in local tick populations are considerably lower. CDC surveillance data through 2025 confirms that the Lyme bacterium has been found in blacklegged tick populations across a wide swath of the eastern United States and in western blacklegged ticks along the Pacific coast, but the density of infected ticks is not uniform.
The geographic range of infected ticks has also been expanding. Areas that were once considered low-risk now have established populations of blacklegged ticks carrying the Lyme bacterium, which means the old advice of “you only need to worry in New England” no longer holds.
A Bite Doesn’t Guarantee Infection
Even if the tick biting you is infected, transmission isn’t instant. The Lyme bacterium lives in the tick’s gut and needs time to migrate to its salivary glands before it can enter your body. In most cases, an infected tick must be attached for more than 24 hours before transmission occurs. Removing a tick within that first 24-hour window greatly reduces your chance of getting Lyme disease.
So the real-world math works like this: in a high-risk area, roughly 1 in 4 nymph bites involves an infected tick, but only a fraction of those will result in transmission if you’re checking for ticks regularly. The combination of infection rate, attachment time, and how quickly you find the tick all factor into your actual risk.
Deer Ticks Carry More Than Just Lyme
Lyme gets the most attention, but deer ticks can carry several other pathogens at the same time. A Pennsylvania study analyzing ticks that tested positive for the parasite that causes babesiosis found that 65% of those ticks were co-infected with at least one other pathogen. Among them, about 61% also carried the Lyme bacterium, and roughly 10% carried the agent responsible for anaplasmosis. About 7.5% were carrying all three.
This matters because co-infections can complicate symptoms and make diagnosis harder. A single tick bite can potentially deliver more than one illness. Symptoms like high fever, severe fatigue, or abnormal blood counts alongside the classic signs of Lyme may point to a co-infection. The same preventive measures that protect against Lyme, prompt tick removal and regular body checks, protect against these other infections too.
How to Lower Your Risk in Practice
The most effective thing you can do is shorten the window of attachment. A full-body tick check after spending time outdoors is the single most important habit, especially during nymph season from May through July. Pay close attention to hidden spots: behind the ears, along the hairline, in the armpits, behind the knees, and around the waistband.
If you find a tick, use fine-tipped tweezers to grasp it as close to the skin as possible and pull straight up with steady pressure. Don’t twist, crush, or try to smother it. After removal, clean the bite area with rubbing alcohol or soap and water. Note the date you found it. If a circular rash appears in the following days to weeks, or you develop fever, fatigue, or joint pain, that timeline will be useful information for your doctor.
Treating clothing and gear with permethrin, or using EPA-registered insect repellents containing DEET or picaridin on exposed skin, adds another layer of protection. Tucking pants into socks looks ridiculous and works well. Showering within two hours of coming indoors has also been shown to help wash off ticks that haven’t yet attached.