Lyme disease spreads through the bite of infected blacklegged ticks, sometimes called deer ticks. There is no other confirmed way to contract it. An estimated 476,000 people are diagnosed and treated for Lyme disease each year in the United States, making it the most common tick-borne illness in the country.
The Only Proven Route: Tick Bites
The bacteria that cause Lyme disease live inside certain species of ticks. When an infected tick bites you and stays attached long enough to feed, the bacteria move from the tick’s gut into your bloodstream. In the eastern United States, the blacklegged tick (sometimes called the deer tick) is the carrier. Along the Pacific coast, a closely related species, the western blacklegged tick, fills the same role. No other type of tick in North America transmits the disease.
Two species of bacteria cause Lyme disease in the U.S. The vast majority of cases come from one species, though a second, rarer species identified in the upper Midwest can also infect humans and has been found circulating in the blood of patients during active illness. In Europe, several additional bacterial species cause the disease, which is why Lyme can look slightly different depending on where in the world someone is infected.
You cannot get Lyme disease from another person. It does not spread through sexual contact, kissing, blood transfusions, or breast milk. You also can’t catch it from touching a deer, eating venison, or being near someone who has it.
Why Most People Never See the Tick
The ticks most likely to give you Lyme disease are nymphs, the juvenile stage of the blacklegged tick. Nymphs are roughly the size of a poppy seed. At that scale, they’re easy to miss entirely, even on bare skin. Many people diagnosed with Lyme disease never recall being bitten.
Adult female ticks also carry the bacteria, but they’re closer to the size of a sesame seed, making them easier to spot and remove before transmission occurs. Male adult ticks rarely bite humans.
The feeding process matters. A tick that has just landed on you and hasn’t started feeding poses very little risk. If the tick’s body is still flat, it likely hasn’t been attached long enough to transmit the bacteria. A tick that is visibly swollen or engorged with blood has been feeding for a longer period, and the risk of transmission is significantly higher.
When and Where Exposure Happens
Tick encounters follow a seasonal pattern. Nymphs are most active from April through July, which is when the majority of Lyme disease cases begin. Adult ticks are most active in early spring and again in fall, creating a secondary window of risk.
Blacklegged ticks need moisture to survive. They are highly vulnerable to drying out. Research shows that at humidity levels below about 82%, nymphal ticks struggle to absorb enough moisture from the air and can die within 48 hours. This is why ticks concentrate in shaded, humid environments rather than open, sunny areas. Studies have consistently found that tick survival is higher in wooded and forest-edge habitats compared to open fields.
The leaf litter on a forest floor is the tick’s home base. Ticks rest in the damp lower layers of fallen leaves, then climb up onto low vegetation or the upper layer of leaf litter to wait for a host to brush past. On hot, dry days, they retreat deeper into the litter to rehydrate, which is why tick activity can actually drop during the warmest parts of a summer afternoon. You’re most likely to pick up a tick while walking through wooded trails, brushing against tall grass at the edge of a forest, gardening near tree lines, or sitting on logs or leaf-covered ground.
How Pets Bring Ticks to You
Dogs and cats that spend time outdoors can carry unattached ticks into your home. A tick that hasn’t yet bitten your pet may crawl off onto furniture, bedding, or clothing and eventually find its way to you. This is a genuinely common route of human exposure, especially for people who don’t spend much time outdoors themselves but live with pets that do.
Checking your pets daily for ticks helps protect both them and you. Running your fingers through their fur, paying attention to ears, neck, and between toes, can catch ticks before they detach and wander. Tick prevention products for pets reduce this risk substantially.
The Rash and Other Early Signs
The most recognizable sign of Lyme disease is an expanding skin rash called erythema migrans, which appears in over 70% of infected people. While many people picture a neat bullseye pattern, the rash actually takes many forms. It can appear as a uniformly red oval, a bluish patch without any central clearing, an expanding ring with a crusty center, or a red-blue lesion with partial clearing. Some people develop multiple rashes in different locations, which signals the infection has begun to spread.
The rash typically appears at the site of the bite and expands over days to weeks. Because the tick that caused it was likely a tiny nymph that fell off unnoticed, the rash may appear in a spot you never knew was bitten. About 30% of people with Lyme disease never develop a visible rash at all, which is one reason the disease is frequently missed or diagnosed late.
What to Do After a Tick Bite
If you find an attached tick, remove it as soon as possible using fine-tipped tweezers. Grasp the tick as close to your skin as you can and pull straight up with steady pressure. Don’t twist, crush, or try to burn the tick off.
After removal, note whether the tick appeared flat or engorged. A flat tick is unlikely to have transmitted the bacteria. An engorged tick warrants more concern, especially if you’re in an area where Lyme disease is common. Preventive treatment is most effective when started within 72 hours of removing the tick. The incubation period for Lyme disease is at least three days, so that 72-hour window exists for a reason.
If you’re unsure what kind of tick bit you, that alone doesn’t rule out the need for preventive care. Tick identification is difficult even for experienced people, and a healthcare provider can still consider preventive treatment when the tick species is uncertain. Over 89,000 cases were reported to the CDC in 2023 alone, and actual numbers are likely several times higher, so taking a tick bite seriously in a high-risk area is reasonable.
High-Risk Regions
Lyme disease is concentrated in two main areas of the United States. The Northeast and upper Midwest carry the highest burden, stretching from Maine down through Virginia and west through Minnesota and Wisconsin. A smaller number of cases occur along the northern Pacific coast. If you live in, travel to, or have pets that roam in these regions, your risk of encountering an infected tick is meaningfully higher than in other parts of the country.
The geographic range of blacklegged ticks has been expanding over recent decades. Counties that had no established tick populations 20 years ago now report consistent surveillance findings. If you’ve lived in an area for years without worrying about ticks, it’s worth reassessing, especially if you’ve noticed more ticks on your pets or in your yard than in previous years.