The percentage of ticks carrying Lyme disease bacteria ranges from nearly zero to over 50%, depending on where you live and the life stage of the tick. In the northeastern United States, about half of adult blacklegged ticks are infected. On the West Coast, the number drops dramatically, often to low single digits.
Not All Ticks Carry Lyme
Only blacklegged ticks (also called deer ticks) transmit Lyme disease in the United States. The species found in the eastern half of the country is Ixodes scapularis, while the western blacklegged tick, Ixodes pacificus, lives along the Pacific coast. Other common ticks, like the American dog tick, the lone star tick, and the Rocky Mountain wood tick, do not transmit the Lyme bacterium at all. So if you find a tick on you, the species matters as much as the location.
Infection Rates in the Northeast
A 2025 Dartmouth study published in Parasites & Vectors tested blacklegged ticks across five northeastern states: Connecticut, New York, New Hampshire, Vermont, and Maine. The results were striking. Roughly 50% of adult blacklegged ticks carried the Lyme bacterium, while up to 25% of nymphal ticks (the younger, smaller life stage) were infected.
These are the highest infection rates in the country, and they align with disease patterns. The Northeast and upper Midwest account for the vast majority of Lyme disease cases reported each year. If you live in or visit these regions, encountering an infected tick is a realistic possibility, not a rare event.
Infection Rates on the West Coast
Western blacklegged ticks carry Lyme bacteria at much lower rates. Studies in California have found infection prevalence in the low single digits for most counties, and some collections found no infected ticks at all. Research published in Applied and Environmental Microbiology examined ticks across multiple California counties and found that infection rates varied by habitat type, with woodland areas generally posing more risk than coastal grasslands or chaparral.
The reasons for this difference are partly ecological. In the West, the ticks’ primary hosts during their juvenile stages are western fence lizards, which are poor reservoirs for the Lyme bacterium. The lizards’ blood actually kills the bacteria inside feeding ticks. In the Northeast, juvenile ticks feed primarily on white-footed mice, which are excellent Lyme reservoirs and efficiently pass the infection along.
Why Tick Life Stage Matters
Adult ticks are easier to spot (roughly the size of a sesame seed when unfed) and tend to be most active in fall and early spring. Nymphal ticks, active in late spring and summer, are about the size of a poppy seed. Despite their lower infection rates compared to adults, nymphs cause the majority of Lyme disease cases in humans. Their tiny size makes them far harder to detect, so they’re more likely to feed long enough to transmit the bacterium.
The CDC notes that a tick generally needs to be attached for a sustained period before transmission occurs. Finding and removing a tick quickly reduces your risk significantly, which is why daily tick checks after spending time outdoors are one of the most effective prevention strategies. Ticks that haven’t yet become engorged (visibly swollen with blood) are less likely to have been attached long enough to transmit infection.
Infection Rates by Region at a Glance
- Northeast and upper Midwest: Up to 50% of adult blacklegged ticks and up to 25% of nymphs carry the Lyme bacterium.
- Mid-Atlantic states: Rates are moderately high, though they vary by county and habitat.
- West Coast: Generally low single-digit percentages in western blacklegged ticks, with some areas showing no infected ticks in sample collections.
- South and Mountain West: Blacklegged ticks are less common or absent, and Lyme transmission risk is minimal in most areas.
What This Means in Practice
Knowing the infection rate in your area helps you gauge your actual risk. If you’re hiking in Connecticut and find an attached adult blacklegged tick, there’s roughly a coin-flip chance it carries the Lyme bacterium. If you find a western blacklegged tick in a California park, the odds are much lower. Neither scenario guarantees infection, because the tick also needs to feed long enough to transmit the bacteria, but the starting probabilities are very different.
Infection rates also shift over time. Tick populations are expanding northward and into new areas as winters become milder, and the percentage of infected ticks in a given region can rise as deer and mouse populations change. Areas that were once considered low risk, including parts of northern New England and southern Canada, now report substantial infection rates. Checking current local surveillance data from your state health department gives you the most accurate picture for your specific area.