Reported cases have risen, but the real number is likely higher
The Centers for Disease Control and Prevention (CDC) recorded approximately 476,000 cases of Lyme disease in the United States between 2004 and 2016. That works out to roughly 30,000 cases per year on average during that period. However, the CDC itself estimates the actual number of infections is substantially higher—somewhere between 300,000 and 400,000 per year—because many cases go unreported, misdiagnosed, or undiagnosed entirely.
The gap between reported and actual cases exists for several reasons. Some people never see a doctor for symptoms they attribute to flu or fatigue. Others are tested but receive a false negative result, particularly early in infection when antibodies have not yet developed. Still others are diagnosed with Lyme disease but the diagnosis is never formally reported to public health authorities. This means the official CDC count is a floor, not a ceiling.
Key Takeaways
- The CDC reports roughly 30,000 Lyme disease cases per year, but estimates the true number is 10 to 12 times higher, around 300,000 to 400,000 annually.
- Cases are concentrated in the Northeast and Upper Midwest, with the highest rates in states like Connecticut, Delaware, and New Jersey.
- Risk varies dramatically by geography—living in a high-incidence area increases your exposure to infected ticks far more than living elsewhere.
- Most people who are infected and treated early recover fully, but delayed diagnosis can lead to complications affecting joints, the nervous system, and the heart.
Where Lyme disease is most common
Lyme disease is not evenly distributed across the country. The vast majority of cases occur in the Northeast and Upper Midwest. States with the highest reported rates include Connecticut, Delaware, New Jersey, New York, Pennsylvania, Rhode Island, and Massachusetts. In some counties within these states, the infection rate in ticks can exceed 50 percent.
The West Coast has a smaller but growing number of cases, primarily in California, Oregon, and Washington. The tick species that carries Lyme disease in the West (Ixodes pacificus) is less efficient at transmitting the bacteria than its Eastern cousin (Ixodes scapularis), so infection rates are lower even in areas where the tick is common.
If you live outside these high-incidence regions, your risk of contracting Lyme disease from a local tick is much lower. However, you can still contract it while traveling or hiking in endemic areas, and cases have been documented in every state except Hawaii.
How age and season affect your risk
Lyme disease occurs in people of all ages, but the age distribution is bimodal—meaning it peaks in two different groups. Children aged 5 to 9 and adults aged 60 and older have the highest reported rates. This pattern reflects exposure: young children spend time outdoors in tick habitats, and older adults often engage in outdoor recreation like hiking and gardening.
The risk of tick exposure is highest from May through October, with peak transmission occurring in June and July when nymphal ticks (the smallest, hardest-to-spot stage) are most active. However, ticks can transmit Lyme disease year-round in milder climates, and adult ticks remain active on warm winter days even in cold regions.
How likely infection is after a tick bite
Not every tick bite results in Lyme disease, even in high-incidence areas. The tick must be infected with the bacterium Borrelia burgdorferi, and it must remain attached long enough to transmit the infection. Research suggests transmission typically requires 24 to 48 hours of attachment, though some evidence suggests it can occur in as little as 16 hours.
In endemic areas where 20 to 50 percent of ticks carry the bacteria, the overall risk of infection from a single tick bite is still relatively low—roughly 1 to 3 percent if the tick is removed promptly. The risk rises significantly if the tick goes unnoticed for several days. This is why prompt removal and careful inspection after outdoor activity are important protective steps.
Trends over the past two decades
Reported Lyme disease cases have increased substantially since the 1990s. The CDC documented approximately 10,000 cases per year in the early 1990s, compared to the current average of 30,000 per year. This increase reflects several factors: geographic expansion of the tick population northward and westward, longer warm seasons that extend the tick activity window, and increased awareness leading to more diagnoses.
The trend is not uniform across regions. Some areas have seen dramatic increases, while others have remained relatively stable. The geographic range of infected ticks has expanded, particularly in the Northeast and Midwest, bringing Lyme disease to areas where it was previously rare or absent.
Why reported numbers undercount actual infections
The CDC's estimate that actual cases are 10 to 12 times higher than reported cases comes from multiple sources of undercount. Passive surveillance—relying on doctors to report cases—captures only a fraction of infections. Some people recover without ever seeking medical care. Others are treated empirically for Lyme disease based on symptoms and exposure history without confirmatory testing.
Testing itself introduces gaps. The standard two-tier testing protocol (ELISA followed by Western blot) can miss infections in the first week or two of illness, before antibodies develop. Some people are tested only once, and a single negative test does not rule out Lyme disease if performed too early. Additionally, not all positive tests are reported to public health authorities, particularly in states without mandatory reporting requirements.
What this means for your personal risk
Your actual risk of contracting Lyme disease depends almost entirely on where you live and how much time you spend in tick habitat. If you live in Connecticut, New Jersey, or another high-incidence state and spend time outdoors in wooded or brushy areas, your risk is substantially higher than someone living in Arizona or Florida. If you live in a low-incidence area but rarely venture into tick habitat, your risk is very low regardless of national statistics.
The prevalence data matters less than your specific exposure. A person in rural Pennsylvania who hikes regularly faces a different risk profile than someone in suburban New Jersey who stays in mowed yards. Understanding your local tick population and your personal outdoor habits gives you better information than national averages.
Frequently Asked Questions
Is Lyme disease becoming more common?
Yes, reported cases have increased from roughly 10,000 per year in the 1990s to about 30,000 per year now. The geographic range of infected ticks has also expanded. However, some of this increase reflects better awareness and diagnosis rather than purely increased infection rates.
What percentage of ticks carry Lyme disease?
This varies dramatically by location. In some high-incidence counties in the Northeast, 20 to 50 percent of ticks carry the bacteria. In low-incidence areas, the rate may be less than 1 percent. Your local health department or state health lab can often tell you the infection rate in ticks in your region.
If I find a tick on me, what are the odds I have Lyme disease?
Even in endemic areas, a single tick bite carries a low risk—roughly 1 to 3 percent if the tick is removed promptly. The risk increases if the tick has been attached for more than 24 hours. Prompt removal and careful inspection after outdoor activity reduce your risk substantially.
Can I get Lyme disease outside the Northeast?
Yes. Cases have been documented in every state except Hawaii. The West Coast has a growing number of cases, and infected ticks are spreading to new areas. However, the absolute number of cases outside the Northeast and Upper Midwest remains much lower.