Lyme disease is a bacterial infection spread through the bite of infected ticks. It’s the most common tick-borne illness in the United States, with an estimated 476,000 people diagnosed and treated for it each year. Caught early, it’s highly treatable with antibiotics. Left untreated, it can spread to the joints, heart, and nervous system.
What Causes Lyme Disease
Lyme disease is caused by the bacterium Borrelia burgdorferi, which lives inside certain species of ticks. In rare cases, a second species called Borrelia mayonii is responsible. The bacteria enter your bloodstream when an infected tick bites you and stays attached long enough to feed. In most cases, the tick needs to be attached for more than 24 hours before it can transmit the infection, which means prompt tick removal significantly lowers your risk.
Only specific ticks carry Lyme disease. In the eastern United States, the blacklegged tick (sometimes called the deer tick) is the primary carrier and is widely distributed. Along the Pacific coast, particularly in northern California, the western blacklegged tick fills that role. These are small, teardrop-shaped ticks that can be easy to miss on your skin, especially in the nymph stage when they’re roughly the size of a poppy seed.
Where Lyme Disease Is Most Common
Lyme disease is concentrated in the Northeast, mid-Atlantic, and upper Midwest, with additional cases along the northern Pacific coast. Over 89,000 cases were reported to the CDC in 2023, though the true number of infections is much higher since many cases go unreported or are treated without formal reporting. The gap between the roughly 89,000 reported cases and the estimated 476,000 annual diagnoses gives a sense of how widespread the disease actually is.
The Telltale Rash and Early Symptoms
The most recognizable sign of Lyme disease is a spreading rash called erythema migrans, which appears in more than 70% of infected people. It typically shows up 3 to 30 days after the tick bite and expands outward over time, sometimes reaching several inches across. While many people picture a neat bullseye pattern with a red ring and clear center, the rash actually takes many forms. It can appear as a solid red oval, a bluish patch without central clearing, a lesion with a crusty center, or a red-blue spot. Some people develop multiple rashes in different locations, which signals the infection has begun spreading through the body.
Along with the rash, early Lyme disease often brings flu-like symptoms: fever, fatigue, headache, muscle aches, and swollen lymph nodes. These symptoms can be mild enough that some people dismiss them, particularly if the rash never appears or goes unnoticed in a hard-to-see location like the scalp or back.
What Happens if It Spreads
Without treatment, the bacteria can spread beyond the skin to other parts of the body over weeks to months. The joints are a common target, with Lyme arthritis causing severe swelling and pain, particularly in the knees. Neurological symptoms can also develop, including facial palsy (weakness or drooping on one or both sides of the face), numbness and tingling in the hands or feet, and inflammation around the brain and spinal cord that causes headaches and neck stiffness.
In about 1 out of every 100 reported cases, the bacteria reach the heart, a condition called Lyme carditis. This disrupts the electrical signals that coordinate your heartbeat, creating what’s known as heart block. Symptoms include light-headedness, fainting, shortness of breath, palpitations, and chest pain. Lyme carditis is uncommon but serious, and people with this complication typically also have fever, body aches, and sometimes the characteristic rash.
How Lyme Disease Is Diagnosed
Doctors can often diagnose early Lyme disease based on the expanding rash alone, especially if you live in or recently visited an area where Lyme is common. When blood testing is needed, the standard approach uses two tiers: an initial screening test that looks for antibodies your immune system produces against the bacteria, followed by a more specific confirmation test if the first one is positive or borderline.
Timing matters with blood tests. Your body needs time to build antibodies, so tests taken within the first two weeks of infection may come back negative even if you’re infected. If that happens and Lyme disease is still suspected, retesting with a new blood sample 7 to 14 days later is recommended. This is one reason the rash is so diagnostically valuable: it often appears before blood tests can detect the infection.
Treatment and Recovery
Early Lyme disease is treated with a course of oral antibiotics lasting 10 to 14 days. When the disease has spread to the joints, nervous system, or heart, treatment may be longer or require intravenous antibiotics depending on severity. Most people recover fully with appropriate treatment, and shorter courses are generally preferred to reduce side effects.
Some people, however, continue to experience fatigue, body aches, or difficulty thinking after completing treatment. This is called Post-Treatment Lyme Disease Syndrome, and its cause remains unknown. Similar lingering symptoms have been reported after other types of infections as well. Careful studies conducted over the past two decades in the U.S. and Europe have generally found that extended courses of antibiotics are no better than placebo for resolving these prolonged symptoms. The CDC specifically discourages the term “chronic Lyme disease” because it implies an ongoing bacterial infection when the actual cause of persistent symptoms hasn’t been identified.
Prevention After a Tick Bite
If you find a tick on your body, remove it as soon as possible with fine-tipped tweezers by grasping it close to the skin and pulling straight up. The sooner you remove it, the lower your risk. A flat, unfed tick is unlikely to have transmitted the bacteria. An engorged tick, one that’s visibly swollen with blood, carries a higher risk.
In areas where Lyme disease is common, a single preventive dose of an antibiotic can be given after a tick bite. This works best within 72 hours of tick removal, since the Lyme disease incubation period is at least three days. Not everyone who gets bitten needs this preventive treatment. It’s most relevant when the bite happened in a region where Lyme is prevalent and the tick appears to have been attached long enough to feed.
Reducing Your Risk
The most effective prevention starts with avoiding tick bites in the first place. When spending time in wooded or grassy areas, wearing long pants tucked into socks and using insect repellent on exposed skin creates a meaningful barrier. Treating clothing and gear with permethrin adds another layer of protection. After coming indoors, doing a full-body tick check is one of the simplest and most effective steps you can take. Pay close attention to hidden areas: the scalp, behind the ears, under the arms, around the waist, and behind the knees. Showering within two hours of being outdoors helps wash off unattached ticks and gives you an opportunity to spot ones that have latched on.