Lyme disease is caused by a type of corkscrew-shaped bacteria called Borrelia burgdorferi, spread to humans through the bite of an infected blacklegged tick. In rare cases, a second species called Borrelia mayonii can also cause the disease. No other route of transmission has been confirmed: you can’t catch Lyme from another person, from your pets, or from food.
The Bacteria Behind the Infection
Borrelia burgdorferi belongs to a group of bacteria known as spirochetes, named for their distinctive spiral shape. That shape lets them burrow efficiently through skin and connective tissue, which is part of what makes Lyme disease so effective at spreading through the body once it takes hold. In the United States, B. burgdorferi is responsible for the vast majority of cases. B. mayonii, identified more recently in the upper Midwest, causes similar symptoms but can also produce unusually high levels of bacteria in the bloodstream, sometimes triggering nausea and vomiting that aren’t typical of standard Lyme.
How Ticks Transmit the Bacteria
The bacteria live inside the gut of infected blacklegged ticks. When a tick latches onto your skin and begins feeding, the bacteria slowly migrate from the tick’s gut to its salivary glands, then pass into your body through the tick’s saliva. This migration takes time, which is why the duration of attachment matters so much: a tick generally needs to be attached for more than 24 hours before transmission occurs. If you find and remove a tick within that window, your risk drops significantly.
Two tick species carry Lyme disease in the U.S. The blacklegged tick (sometimes called the deer tick) is widely distributed across the eastern half of the country and is the primary vector. The western blacklegged tick fills a similar role along the Pacific coast, particularly in northern California. Both species are small, especially in their nymphal (juvenile) stage, when they’re roughly the size of a poppy seed and easy to miss on the skin. Nymphs are responsible for most human infections because they’re so hard to spot.
The Animal Reservoir That Keeps the Cycle Going
Ticks don’t generate Borrelia bacteria on their own. They pick up the bacteria by feeding on infected wildlife, then pass it along during their next blood meal. The white-footed mouse is the most important reservoir host in the eastern United States. These mice are extremely efficient at maintaining the infection: in one study, 91% of mice exposed to infected tick nymphs developed active infections, and about 75% of larval ticks that later fed on those mice became infected themselves.
Deer also play a role, though not by carrying the bacteria directly. Adult blacklegged ticks feed heavily on deer, so large deer populations support large tick populations. Other animals like chipmunks, shrews, and some bird species can also serve as reservoirs, but the white-footed mouse is the linchpin of the cycle in most regions where Lyme is common.
Where and When Risk Is Highest
Lyme disease is heavily concentrated in the Northeast, mid-Atlantic, and upper Midwest, with a smaller pocket along the northern Pacific coast. Over 89,000 cases were reported to the CDC in 2023, but the true number is likely much higher. Estimates based on insurance records and other data suggest roughly 476,000 people may be diagnosed and treated for Lyme each year in the United States.
Ticks thrive in specific environmental conditions. They need high humidity to survive, at least 80% relative humidity at ground level, which is why they concentrate in shaded, wooded areas with thick leaf litter. Leaf litter insulates ticks from temperature extremes and traps moisture near the soil. In one study, ticks in plots where leaf litter was left intact survived at significantly higher rates than those where it was removed. The edges of woodlands, brushy areas, and spots near stone walls tend to harbor the highest numbers of questing nymphs (ticks actively waiting on vegetation for a host to pass by).
Peak transmission season runs from late spring through summer, roughly May through August, when nymphal ticks are most active and people spend the most time outdoors. Fall brings a second, smaller wave of risk from adult ticks, which are larger and easier to spot but still capable of transmitting the bacteria.
What Happens After a Bite
Once the bacteria enter your skin, the first sign is usually a expanding red rash called erythema migrans, which appears in over 70% of people with Lyme disease. It typically shows up 3 to 30 days after the bite and gradually expands over several days, sometimes developing a central clearing that creates the well-known “bull’s-eye” pattern. The rash isn’t always a perfect target shape, though. It can be uniformly red, oval, or irregularly shaped, and it’s rarely itchy or painful.
Early symptoms beyond the rash tend to mimic the flu: fatigue, headache, fever, muscle aches, and swollen lymph nodes. If left untreated, the bacteria can spread to the joints, heart, and nervous system over weeks to months, causing more serious problems like facial paralysis, severe joint swelling (particularly in the knees), heart palpitations from inflammation of the heart tissue, and nerve pain or numbness in the hands and feet.
How Lyme Disease Is Diagnosed
Diagnosis relies on a combination of symptoms, exposure history, and blood testing. The standard approach uses a two-step blood test: an initial screening test looks for antibodies your immune system produces against the bacteria, and if that comes back positive or borderline, a second, more specific test confirms the result. The catch is that antibodies take time to develop, so testing too early (within the first couple of weeks after a bite) can produce a false negative. If you have the characteristic rash and a plausible tick exposure, treatment typically starts based on clinical judgment rather than waiting for lab results.
Early treatment with antibiotics is highly effective. Most people recover fully within a few weeks. Delayed treatment makes recovery slower and increases the chance of lingering symptoms like fatigue and joint pain, which is why recognizing the early signs matters.
Reducing Your Exposure
Since the entire chain of Lyme disease starts with a tick bite, prevention comes down to avoiding bites and removing ticks quickly. When spending time in wooded or grassy areas, wearing long pants tucked into socks and treating clothing with permethrin (a tick-repelling spray) significantly reduces the chance of a tick reaching your skin. DEET or picaridin applied to exposed skin adds another layer of protection.
After coming indoors, a thorough tick check is your most reliable safeguard. Pay close attention to the scalp, behind the ears, underarms, groin, and behind the knees. Showering within two hours of being outdoors has been associated with lower infection rates, likely because it dislodges ticks that haven’t yet attached. Tossing clothes in a hot dryer for 10 minutes kills any ticks hitching a ride on fabric. If you find an attached tick, remove it with fine-tipped tweezers by grasping as close to the skin as possible and pulling straight out with steady pressure.