Lyme disease is not caused by a virus. It is a bacterial infection, spread through the bite of infected blacklegged ticks. The bacterium responsible, Borrelia burgdorferi, belongs to a group called spirochetes, named for their slim, spiral shape. This distinction matters because it determines how the disease is diagnosed, treated, and prevented.
Why Lyme Disease Was Once Mistaken for a Virus
When Lyme disease was first identified in rural Connecticut in 1975, physicians actually suspected a virus was behind the outbreak. Without knowing the true cause, attempts to understand the disease and develop treatments went nowhere. It wasn’t until 1981 that NIH researcher William Burgdorfer discovered spiral-shaped bacteria in the midguts of deer ticks collected from forests near the affected communities. His 1982 paper confirmed these bacteria, passed to humans through tick bites, were the real cause. The organism was named Borrelia burgdorferi in his honor.
The confusion was understandable. Early Lyme symptoms like fever, fatigue, headache, and muscle aches overlap with many viral infections. But the bacterial nature of the disease turned out to be good news for patients: unlike viruses, bacteria can be killed with antibiotics.
How Borrelia Bacteria Differ From Viruses
Borrelia burgdorferi is a living organism with its own cell membranes and internal structures. Its cells have both an outer and inner membrane, similar to other bacteria, though its outer membrane is unusual in composition. Each cell has a bundle of 7 to 11 helically shaped internal flagella that give it a flat, undulating wave-like appearance and allow it to corkscrew through tissue. Viruses, by contrast, are not cells at all. They are packets of genetic material that hijack your own cells to reproduce.
This difference is why antibiotics work against Lyme disease. Antibiotics target bacterial structures and processes that viruses simply don’t have. The most commonly used antibiotics for Lyme are doxycycline, amoxicillin, and cefuroxime axetil. Early treatment is highly effective and typically leads to a quick recovery. Antiviral medications would do nothing against Borrelia bacteria.
Ticks Can Carry Viruses Too
Part of the confusion around Lyme disease may stem from the fact that ticks transmit both bacterial and viral diseases. Powassan virus, for example, is a genuine tick-borne viral infection that can affect the central nervous system and cause severe inflammation of the brain and its surrounding membranes. Unlike Lyme, Powassan can be transmitted within just 15 minutes of a tick attaching to your skin, and there is no specific medication to treat it. Doctors can only manage symptoms.
Lyme disease, by comparison, requires the tick to be attached for more than 24 hours before the bacterium typically transfers. That longer window is one reason prompt tick removal is so effective at preventing Lyme infections. It also highlights a key practical difference: bacterial tick-borne diseases generally have targeted treatments, while viral ones often do not.
Recognizing Lyme Disease Symptoms
The hallmark sign of Lyme disease is the erythema migrans rash, which appears in about 70 to 80 percent of infected people. It typically shows up 3 to 30 days after a tick bite, with an average of about 7 days. The rash expands gradually, sometimes reaching 12 inches or more across, and may feel warm to the touch but is rarely itchy or painful. While it sometimes develops the well-known “bull’s-eye” pattern of clearing in the center as it grows, it often does not. It can appear on any area of the body.
Even without the rash, early Lyme disease can cause fever, chills, headache, fatigue, muscle and joint aches, and swollen lymph nodes. These flu-like symptoms are another reason people sometimes assume they have a virus rather than a bacterial infection.
If Lyme disease goes untreated, it can progress over days to months into more serious problems. These include severe headaches and neck stiffness, facial palsy (drooping on one or both sides of the face), arthritis with significant joint pain and swelling (particularly in the knees), heart palpitations or irregular heartbeat, nerve pain, and shooting pains or tingling in the hands and feet. In some cases, it can cause inflammation of the brain and spinal cord.
How Lyme Disease Is Diagnosed
Because Lyme is bacterial, diagnosis relies on detecting your immune system’s response to the bacteria through blood tests, not viral testing methods. The CDC recommends a two-step process. The first step is a screening blood test. If that result is negative, no further testing is needed. If it comes back positive or borderline, a second confirmatory test is performed on the same blood sample. The overall result is only considered positive when both steps are positive.
These tests look for two types of antibodies your body produces in response to the infection. One type, IgM, appears early in the illness, while the other, IgG, develops later. A positive IgM result is only considered meaningful if you’ve been sick for 30 days or less, since it can produce misleading results after that window.
How Common Lyme Disease Is
Lyme disease is the most common tick-borne illness in the United States. Over 89,000 cases were reported to the CDC in 2023, but the actual number is far higher. Estimates using broader methods suggest approximately 476,000 people may be diagnosed and treated for Lyme disease each year in the U.S. The gap between reported and estimated cases reflects the fact that many infections are diagnosed and treated without ever being formally reported to public health authorities.
Preventing Tick Bites
Since Lyme disease requires a tick bite to spread (it doesn’t pass person to person like a virus), prevention focuses on avoiding tick attachment. Insect repellents containing DEET, picaridin, or oil of lemon eucalyptus provide reasonably long-lasting protection against ticks. Higher concentrations of the active ingredient generally offer longer protection, while products with less than 10 percent concentration may only work for one to two hours. Heat, sweat, water exposure, and physical activity all reduce how long any repellent remains effective.
Checking your body for ticks after spending time outdoors remains one of the simplest and most effective prevention strategies. Because the Lyme bacterium generally needs more than 24 hours of tick attachment to transmit, finding and removing ticks promptly can prevent infection even if the tick was carrying Borrelia. In areas where Lyme disease is common, a single preventive dose of doxycycline after a tick bite may further lower the risk of developing the disease.