Lyme disease is the most common tick-borne illness in the United States, and its footprint keeps growing. The CDC estimates that about 476,000 Americans are diagnosed and treated for Lyme each year.
In April 2026 the agency reported that emergency-room visits for tick bites were running at their highest weekly levels since 2017 across most of the country.
The infection is caused by Borrelia burgdorferi, a bacterium spread through the bite of infected black-legged ticks. Caught early, it usually clears with a short course of oral antibiotics.
Left untreated, it can move from the skin to the joints, heart and nervous system within weeks to months. Here are eight warning signs to know across the three stages of the illness.
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The expanding rash, and why it fools people
The most recognizable early sign is erythema migrans, a slowly expanding red patch that shows up at the bite site. According to the CDC, it appears 3 to 30 days after a tick bite, with an average of about seven days.
It can expand to 12 inches (30 cm) or more across the skin. The catch is that it does not always look like a textbook bull's-eye.
The CDC's own rash gallery documents solid red plaques, oval lesions, bluish patches and rashes with a crusted center. Any expanding rash after possible tick exposure is worth taking seriously, even without the classic ring.
Fever, headache and fatigue in the first weeks
The rash is often paired with flu-like symptoms. The CDC states that "fever, chills, headache, fatigue, muscle and joint aches, and swollen lymph nodes may occur" in the first days and weeks.
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That overlap is why Lyme is one of the tick-borne infections most often mistaken for the flu in its earliest days.
Signs the infection has spread
If Lyme is not treated in the first weeks, the bacteria can begin to travel. The CDC lists severe headaches and neck stiffness, along with facial palsy, a droop on one or both sides of the face.
Other later-stage signs include heart palpitations or an irregular heartbeat, and episodes of dizziness or shortness of breath. Bell palsy affecting the facial nerve is seen in about 5 percent of patients, according to a review in StatPearls hosted by the NIH's National Library of Medicine.
The heart involvement, known as Lyme carditis, is uncommon but can cause dangerous rhythm problems that need urgent care.
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Late-stage joint and nerve symptoms
Months in, the infection can settle into the joints and nervous system. The CDC describes arthritis with severe joint pain and swelling, particularly in the knees and other large joints.
Late-stage Lyme can also involve inflammation of the brain and spinal cord, and shooting pains, numbness or tingling in the hands or feet. Some patients continue to have persistent joint inflammation even after antibiotics, the StatPearls review notes.
Why testing can miss it in the first weeks
The standard Lyme blood test looks for antibodies, which the immune system needs time to build. Serologic testing is unreliable during the first few weeks of infection, according to the StatPearls review, so a negative result early on does not rule Lyme out.
When the rash and a plausible tick exposure are both present, doctors are advised to treat based on the clinical picture rather than wait for a positive test.
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When to see a doctor
The CDC's guidance is straightforward. Seek medical attention if you find an attached tick, if you notice an expanding rash within a month of possible tick exposure, or if flu-like symptoms show up in tick season without another obvious cause.
Most infections respond to 10 days to four weeks of doxycycline, amoxicillin or cefuroxime axetil. Early antibiotic treatment can prevent the heart, joint and nerve problems that follow untreated Lyme.
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This article is made and published by Jesper Bengtson, who may have used AI in the preparation.