Lyme disease starts with a bacterial infection from a tick bite and, if untreated, can spread through your bloodstream to your joints, heart, and nervous system over weeks to months. Around 476,000 people are diagnosed and treated for it each year in the United States. Most people recover fully with a course of antibiotics, but the timeline and severity depend on how quickly the infection is caught.
The First Days and Weeks
Somewhere between 3 and 30 days after a tick bite (7 days on average), the infection announces itself. The most recognizable sign is a spreading red rash called erythema migrans, which shows up in about 70 to 80 percent of infected people. It starts at the bite site and expands gradually, sometimes reaching 12 inches or more across. The rash may feel warm but is rarely itchy or painful. As it grows, the center sometimes clears, creating the well-known “bull’s-eye” pattern, but many Lyme rashes don’t look like a classic bull’s-eye at all. They can appear as a solid red oval or an irregularly shaped patch.
Even without a rash, early Lyme disease often brings fever, chills, headache, fatigue, muscle and joint aches, and swollen lymph nodes. These symptoms overlap with the flu, which is one reason early Lyme gets missed, especially if you didn’t notice a tick on your body.
How the Bacteria Spread Through Your Body
The spiral-shaped bacteria that cause Lyme disease, called Borrelia burgdorferi, don’t stay put at the bite site. Research from Johns Hopkins has shown that within an hour or two, the bacteria use persistent, trial-and-error movements to find tiny gaps in the walls of nearby blood vessels. Once they slip through those junctions and enter the bloodstream, they circulate throughout the body in seconds. From there, they can settle into joints, the heart, and nerve tissue, which is why untreated Lyme disease can affect so many different organ systems.
What Happens If It Goes Untreated
Left alone, Lyme disease doesn’t stay localized. Over days to months, the infection can produce a wide range of symptoms as bacteria reach new tissues:
- Joints: Arthritis with severe pain and swelling, especially in the knees and other large joints. This is one of the most common later-stage symptoms.
- Nervous system: Severe headaches, neck stiffness, facial palsy (a droop on one or both sides of the face), shooting pains, numbness or tingling in the hands and feet, and in some cases, inflammation of the brain and spinal cord.
- Heart: A complication called Lyme carditis, where bacteria enter heart tissue and disrupt the electrical signals that coordinate your heartbeat. This creates varying degrees of “heart block,” meaning the signal between the upper and lower chambers of the heart slows down or stops entirely. Lyme carditis occurs in roughly 1 out of every 100 reported cases, and when it’s severe, the lower chambers of the heart beat at their own slower rhythm. You might feel heart palpitations, dizziness, or shortness of breath.
Additional rashes can also appear on other parts of the body as the bacteria spread, along with intermittent pain in tendons, muscles, and bones. The pattern is often unpredictable. Symptoms can come and go, which sometimes leads people to dismiss them before they worsen.
How Lyme Disease Is Diagnosed
Diagnosis relies on a combination of your symptoms, possible tick exposure, and blood tests that look for antibodies your immune system makes in response to the bacteria. The catch is timing: during the first few weeks of infection, your body may not have produced enough antibodies to show up on a test. Standard blood tests have a sensitivity of only 35 to 60 percent in that early window, meaning they miss a significant portion of true infections. After 4 to 6 weeks, the tests become much more reliable.
This is why doctors often diagnose early Lyme based on the rash alone, without waiting for lab confirmation. If you have a spreading rash and recent tick exposure, that’s typically enough to start treatment. Newer testing methods have improved early detection by about 10 to 20 percent compared to older approaches, but the early window remains the hardest period to confirm the diagnosis through bloodwork.
Treatment and What to Expect
Lyme disease is treated with oral antibiotics, typically for 10 to 14 days in early cases. Most people with early Lyme who complete a course of antibiotics recover fully. When the infection has progressed to later stages involving the joints, heart, or nervous system, treatment may be longer or involve intravenous antibiotics, but the large majority of patients still improve.
One thing that catches people off guard is a temporary worsening of symptoms shortly after starting antibiotics. This is called a Herxheimer reaction, and it happens because dying bacteria release substances that trigger a brief inflammatory response. Symptoms like fever, chills, muscle pain, headache, and a faster heart rate can appear within hours of your first dose. It typically resolves within a day on its own and is actually a sign that the antibiotics are working.
Lingering Symptoms After Treatment
Most people feel better within weeks of finishing antibiotics, but a subset continues to experience fatigue, body aches, or difficulty thinking for months afterward. This is known as Post-Treatment Lyme Disease Syndrome (PTLDS), and its cause remains unknown. The bacteria don’t appear to be actively present anymore, so additional rounds of antibiotics haven’t been shown to help. Researchers suspect the prolonged symptoms may be related to residual inflammation or changes in how the immune system responds after the infection clears.
PTLDS can be frustrating because there’s no definitive test for it and no targeted treatment. If you’re still experiencing symptoms after completing Lyme treatment, the priority is working with a healthcare provider to rule out other possible causes and manage symptoms individually. For most people with PTLDS, symptoms do improve gradually over time, though the timeline varies widely from person to person.
Why Early Detection Matters
The single biggest factor in how Lyme disease plays out is how early it’s caught. People treated during the initial rash stage almost always recover completely within a few weeks. Those who aren’t diagnosed until the bacteria have reached the joints, heart, or nervous system face a longer recovery and a higher chance of lingering effects. Checking your body for ticks after spending time outdoors, especially in wooded or grassy areas, and paying attention to any expanding rash or unexplained flu-like symptoms during tick season gives you the best chance of catching it before it spreads.