Lyme disease is a bacterial infection that can affect your skin, joints, heart, and nervous system if left untreated. It starts at the site of a tick bite and, over weeks to months, the bacteria can spread through your bloodstream into tissues throughout the body. 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.
How the Bacteria Spread Through Your Body
Lyme disease is caused by a corkscrew-shaped bacterium called Borrelia burgdorferi, transmitted through the bite of an infected blacklegged tick. After entering the skin, the bacteria can invade a surprisingly wide range of human cells, including those lining blood vessels, skin cells, joint tissue, and nerve cells. What makes this bacterium particularly difficult for your immune system to handle is that it slips inside your cells without killing them. Because the cells appear normal from the outside, the immune system has a harder time detecting the infection.
The bacteria also have a second trick: they can change their shape. Under stressful conditions, such as when the immune system mounts a response, the spiral-shaped bacteria can compress into inactive round forms or cluster into protective colonies. This ability to go dormant and hide inside cells helps explain why Lyme disease can persist and cause problems in multiple organ systems if not treated early with antibiotics.
The Expanding Rash: First Sign of Infection
The earliest and most recognizable effect of Lyme disease is a skin rash at the bite site. This rash gradually expands outward over days, often reaching several inches in diameter. It can take several forms: a classic bull’s-eye pattern with a red ring and central clearing, a solid red oval, a bluish-toned lesion, or a rash with a crusty center. Not everyone develops the textbook target shape, so any expanding rash after time spent in a tick-prone area warrants attention.
Along with the rash, early Lyme disease often causes flu-like symptoms: fatigue, fever, headache, muscle aches, and swollen lymph nodes. These symptoms reflect your immune system responding to the spreading infection. At this stage, the bacteria are still relatively localized, and treatment is most effective.
What Happens to Your Nervous System
If the infection goes untreated, the bacteria can reach the nervous system within weeks, a condition known as neuroborreliosis. The most common neurological effects include numbness, pain, weakness, and visual disturbances. One of the hallmark signs is facial palsy, where the muscles on one or both sides of the face droop, similar in appearance to Bell’s palsy. About 9 out of every 100 reported Lyme disease cases involve facial palsy.
Some people develop meningitis, an inflammation of the membranes surrounding the brain and spinal cord, which causes fever, stiff neck, and severe headache. Others experience radiculopathy, a painful nerve inflammation that can cause shooting pain, tingling, or weakness in the limbs. Roughly 3 out of 100 reported cases involve meningitis or encephalitis, and 4 out of 100 involve radiculopathy. People who don’t receive treatment during the early stages and progress to late-stage disease risk varying degrees of permanent nervous system damage.
Effects on the Heart
In about 1 out of every 100 reported cases, Lyme disease bacteria enter the heart tissue and cause a condition called Lyme carditis. The bacteria interfere with the electrical signals that coordinate your heartbeat, specifically the signals traveling from the upper chambers to the lower chambers. This disruption, called heart block, means the heart’s chambers fall out of sync. The severity can range from a slight delay in the electrical signal to a complete block where the upper and lower chambers beat independently of each other.
What makes Lyme carditis particularly unpredictable is that the degree of heart block can change rapidly, sometimes shifting from mild to severe within hours. Symptoms may include lightheadedness, fainting, shortness of breath, heart palpitations, or chest pain. Lyme carditis is rare but potentially life-threatening, and it typically develops during the early weeks of disseminated infection.
Joint Pain and Swelling
Lyme arthritis typically develops one to a few months after the initial infection if the disease remains untreated. The hallmark is obvious swelling of one or a few joints, most commonly the knees. Other large joints like the shoulder, ankle, elbow, jaw, wrist, and hip can also be affected. The swollen joint may feel warm to the touch and hurt during movement.
One distinguishing feature of Lyme arthritis is that the swelling can come and go, or migrate between joints. It may flare for days or weeks, subside, and then return. Swelling in certain joints, particularly the shoulder, hip, and jaw, can be subtle and harder to notice. Unlike rheumatoid arthritis, which tends to affect small joints symmetrically, Lyme arthritis favors large joints and often appears on just one side of the body.
How Lyme Disease Is Detected
Lyme disease is diagnosed through a combination of symptoms, tick exposure history, and blood testing. The standard approach uses a two-step blood test: the first test screens for antibodies your immune system produces against the bacteria, and if that comes back positive, a second, more specific test confirms the result. There’s an important limitation to be aware of. Your body needs time to build antibodies, so blood tests can come back falsely negative during the first four to six weeks after infection. This is why doctors often diagnose and treat early Lyme disease based on the rash and symptoms alone, without waiting for lab results.
Treatment and Recovery
Most cases of Lyme disease are treated with a 10 to 14 day course of oral antibiotics. People who receive treatment during the early stage of infection, when the rash is present and the bacteria haven’t spread far, usually recover rapidly and completely. In certain situations, a single preventive dose of an antibiotic given shortly after a tick bite in a high-risk area can lower the chance of developing Lyme disease in the first place.
More advanced cases involving the nervous system, heart, or joints may require longer treatment courses, typically two to four weeks of oral antibiotics, though some situations call for intravenous delivery. The vast majority of people treated at any stage improve, but the further the disease has progressed, the longer recovery takes.
When Symptoms Linger After Treatment
Some people continue to experience fatigue, body aches, and difficulty thinking even after completing a full course of antibiotics. This is known as Post-Treatment Lyme Disease Syndrome, and its cause remains unknown. The lingering symptoms can last for weeks to months and significantly affect daily life, particularly the cognitive difficulties that patients often describe as “brain fog.”
Researchers aren’t sure whether these persistent symptoms result from residual damage caused by the infection, an ongoing immune response triggered by the original infection, or some combination of factors. What is clear is that additional courses of antibiotics haven’t been shown to speed recovery in these cases, suggesting the problem isn’t an active bacterial infection. Most people with lingering symptoms do improve over time, though the timeline varies widely from person to person.