Lyme disease is a bacterial infection spread through the bite of infected blacklegged ticks. It is the most common tick-borne illness in the United States, with an estimated 476,000 cases diagnosed and treated annually. Most people recover fully with a short course of antibiotics, but delayed treatment can lead to serious complications affecting the joints, heart, and nervous system.
How Lyme Disease Spreads
The bacterium responsible for Lyme disease in the United States is a corkscrew-shaped organism called Borrelia burgdorferi. In Europe and Asia, several related species also cause the disease, which is why symptoms can vary somewhat by region. The bacteria live in the guts of blacklegged ticks (also called deer ticks) and transfer to humans during feeding.
Two tick species carry Lyme disease in the U.S. The blacklegged tick (Ixodes scapularis) is found across the Northeast, mid-Atlantic, and north-central states. The western blacklegged tick (Ixodes pacificus) transmits infection along the Pacific Coast. A tick generally needs to be attached for more than 24 hours before the bacteria can pass into your bloodstream. This is why daily tick checks after spending time outdoors are one of the most effective ways to prevent infection. If you find and remove a tick the same day, your risk drops significantly.
Where Lyme Disease Is Most Common
Lyme disease is heavily concentrated in two regions of the country: the Northeast and the upper Midwest. While 30,000 to 40,000 cases are officially reported through surveillance each year, the true number is far higher because many cases go unreported or are diagnosed outside of formal tracking systems.
The geographic reach of the disease has expanded substantially over the past two decades. The number of high-incidence counties in northeastern states increased more than 320% between 1993 and 2012, from 43 counties to 182. In north-central states, the increase was roughly 250%. The center of the northeastern hotspot has shifted westward and northward, moving from the coast of northern New Jersey into east-central Pennsylvania. In the Midwest, northwestern Wisconsin has remained a consistent focal point. A handful of southeastern counties showed high rates during the early years of surveillance but have since dropped off.
The Rash and Other Early Signs
The hallmark of early Lyme disease is a spreading skin rash called erythema migrans, which appears in more than 70% of cases. It typically shows up 3 to 30 days after a tick bite and gradually expands outward from the bite site. You may have heard it described as a “bull’s eye” pattern with a red ring around a clearing center, but that classic look is actually uncommon. One study found only 6% of Lyme rashes had a true bull’s eye appearance. Most are solid red or slightly irregular patches that expand over days, often reaching several inches across.
Beyond the rash, early Lyme disease often feels like a mild flu: fever, fatigue, headache, muscle aches, and swollen lymph nodes. These symptoms can be easy to dismiss, especially if you never noticed the tick bite. Not everyone develops the rash, which means some people progress to later stages without ever recognizing the initial infection.
What Happens Without Treatment
If Lyme disease goes untreated, the bacteria can spread to other parts of the body over weeks to months, causing progressively more serious problems.
- Joints: Lyme arthritis is one of the most common late-stage complications. It typically causes episodes of severe swelling and pain, most often in the knee, though other large joints can be affected.
- Nervous system: Neurologic Lyme disease can cause facial palsy (a drooping on one side of the face), numbness or tingling in the hands and feet, and inflammation around the brain and spinal cord that leads to headaches and neck stiffness.
- Heart: Lyme carditis occurs in roughly 1 out of every 100 reported cases. The bacteria enter heart tissue and disrupt the electrical signals that coordinate your heartbeat, a condition called heart block. This can cause lightheadedness, fainting, shortness of breath, palpitations, or chest pain. The severity can change rapidly and sometimes requires temporary medical intervention.
How Lyme Disease Is Diagnosed
Diagnosis relies on a combination of symptoms, exposure history, and blood tests. If you have the characteristic expanding rash and recent tick exposure, a doctor can often diagnose Lyme disease on clinical grounds alone, without waiting for lab results.
When blood testing is needed, the standard approach uses a two-step process. The first test screens for antibodies your immune system produces in response to the bacteria. If that initial screen comes back positive or borderline, a second, more specific test confirms the result. One important limitation: your body takes time to produce detectable antibodies, so testing done in the first few days after a tick bite can come back negative even if you are infected. If early results are negative but symptoms persist, retesting after a couple of weeks may be recommended. Antibody levels for Lyme can also remain elevated for months or years after successful treatment, so a positive test does not necessarily mean you have an active infection.
Treatment and Recovery
Most cases of Lyme disease are treated successfully with 10 to 14 days of oral antibiotics. The medications most commonly prescribed are doxycycline, amoxicillin, or cefuroxime. Late-stage disease, particularly Lyme arthritis or neurologic involvement, typically requires a longer course of about four weeks.
The vast majority of people recover fully. However, an estimated 10 to 20% of patients continue to experience symptoms after completing treatment. This is known as post-treatment Lyme disease syndrome, or PTLDS. The lingering symptoms, which can include fatigue, pain, difficulty concentrating, and a general feeling of being unwell, may persist for weeks to months. The exact cause is not fully understood. It could involve residual inflammation, subtle immune system changes triggered by the initial infection, or other factors. Extended courses of antibiotics have not been shown to resolve these symptoms, which is why the condition remains a source of frustration for patients and a focus of ongoing medical attention.
Reducing Your Risk
Prevention centers on avoiding tick bites and removing ticks quickly. When spending time in wooded or grassy areas, especially in high-risk regions from spring through fall, a few strategies make a real difference.
Skin repellents containing DEET, picaridin, IR3535, or oil of lemon eucalyptus create a chemical barrier that discourages ticks from latching on. Treating clothing with permethrin is a separate and complementary approach: permethrin kills ticks on contact with treated fabric and remains effective through multiple washes. Wearing long pants tucked into socks and light-colored clothing (so ticks are easier to spot) adds another layer of protection.
After coming indoors, do a full-body tick check. Pay close attention to hidden spots: behind the ears, along the hairline, under the arms, around the waist, and behind the knees. Ticks in the nymph stage, which are responsible for most Lyme transmission, are roughly the size of a poppy seed, so they’re easy to miss. Showering within two hours of being outdoors can help wash off unattached ticks. If you find an attached tick, remove it by grasping it as close to the skin as possible with fine-tipped tweezers and pulling straight up with steady, even pressure.