Lyme disease is transmitted by black-legged ticks, also known as deer ticks. In the eastern United States, the species is Ixodes scapularis. Along the Pacific coast, its close relative, the western black-legged tick (Ixodes pacificus), carries the same bacteria. No other type of tick in the U.S. transmits Lyme disease to humans.
Where These Ticks Live
The black-legged tick is widely distributed across the eastern United States, with the highest concentrations in the Northeast, mid-Atlantic, and upper Midwest. The western black-legged tick is found along the Pacific coast, particularly in northern California. Together, these two species cover the geographic areas where nearly all U.S. Lyme disease cases originate.
Not every tick in these regions carries the Lyme bacteria. But in high-risk areas of the Northeast, roughly 50% of adult black-legged ticks and up to 25% of nymphs are infected, according to a 2025 Dartmouth study published in Parasites & Vectors. Those are significant odds if you spend time outdoors in tick habitat.
How to Identify a Black-Legged Tick
Black-legged ticks are smaller than the common dog tick, which is the species most people picture when they think of ticks. Unfed adults are about the size of a sesame seed, roughly 2 to 3 millimeters. Adult females have a distinctive red-orange body surrounding a dark brown shield on their back. Adult males are uniformly dark brown.
The nymph stage, which is responsible for a large share of Lyme transmissions, is far harder to spot. Nymphs are about the size of a poppy seed (1.5 millimeters) and translucent to slightly gray or brown. Larvae are even tinier, smaller than 1 millimeter. After feeding, an adult female can swell to about the width of a pencil eraser (6 millimeters) and turn grayish.
If you find a tick on your body, its size and coloring can help you figure out whether it’s a black-legged tick or a less concerning species like a dog tick or lone star tick. You can also save the tick in a sealed bag or rubbing alcohol and bring it to a healthcare provider for identification.
Which Life Stage Is Most Dangerous
Ticks pick up the Lyme bacteria early in life, typically during their larval or nymph stage, when they feed on infected mice and other small animals. They then pass those bacteria to humans during later feedings as nymphs or adults.
Nymphs are particularly risky for two reasons. First, they’re active in late spring and summer, when people spend the most time outdoors in shorts and short sleeves. Second, they’re so small that many people never notice the bite. Adults feed primarily in the fall and early spring, and their larger size makes them easier to find and remove before transmission occurs.
How Long a Tick Must Be Attached
An infected tick generally needs to be attached for more than 24 hours before it can transmit the Lyme bacteria. This is a critical window. Daily tick checks after spending time outdoors, especially in wooded or grassy areas, give you a realistic chance of removing the tick before infection occurs.
To remove a tick safely, use fine-tipped tweezers and grasp it as close to the skin as possible. Pull upward with steady, even pressure. Don’t twist, jerk, or squeeze the tick’s body. After removal, clean the bite area and your hands with rubbing alcohol or soap and water. Flush the tick down a toilet or save it for identification.
Other Infections From the Same Tick
The black-legged tick doesn’t just carry Lyme disease. The same species transmits the bacteria that cause anaplasmosis and the parasite behind babesiosis, among other pathogens. A CDC study that tested over 13,400 Ixodes ticks from 17 states found that coinfections, where a single tick carries the Lyme bacteria alongside another pathogen, were more common than expected in the Northeast.
This means a single tick bite can potentially deliver more than one infection at the same time. If you develop symptoms after a bite, such as fever, chills, body aches, or a rash, mention the tick exposure to your healthcare provider so they can evaluate for multiple tick-borne illnesses, not just Lyme.
Preventive Treatment After a Bite
In areas where Lyme disease is common, a single dose of the antibiotic doxycycline taken shortly after a tick bite may lower the risk of developing the disease. This isn’t recommended for every tick bite, but it’s worth asking about if you were bitten by a black-legged tick that was attached for an extended period in an endemic region.