Detailed findings & methodology:

Lyme disease is relatively new as far as diseases go. While it likely began thousands of years earlier, it was first properly recorded in the United States in the summer of 1975. Dozens of children came down with symptoms that at first appeared to be juvenile arthritis. Two years later, a doctor at Yale published a paper outlining the new tick-borne disease, then called Lyme arthritis.

Lyme disease is concentrated mainly in the Northeastern and mid-Atlantic United States — the blacklegged ticks’ main habitat area. That area comprises a dozen of the 14 states on this list. The Midwest also has a few states with higher than average incidences of Lyme disease.

On the West Coast, particularly northern California, the western blacklegged tick carries Lyme disease. However, no state that is entirely west of the Mississippi River has an above average incidence of Lyme disease.

24/7 Wall St. ranked the states by incidence of Lyme disease, which is based on confirmed reported cases. However, there are several limitations to the data. As the CDC notes, the disease is likely under-reported due to two main reasons. First, because the symptoms of Lyme disease can be so general, and because the current test are more reliable a few weeks after contracting the disease, Lyme disease can be difficult to diagnose. Second, not all states have similar procedures for reporting Lyme disease to the CDC.

Lyme disease tends to affect either children ages 6-9, or adults ages of 40-50, though it is difficult to know why. Dr. Daniel Cameron, an expert on Lyme disease, said the disparity may come from teenagers and young adults underreporting their symptoms. “I’m concerned teenagers don’t always share how sick they are with their parents. … I think they get overlooked.”

There is a huge gap between the states on this list and those that are not. Virginia, the state with the lowest incidence on our list, had 11.6 confirmed cases per 100,000 residents in 2016. The next highest state, Iowa, had just 2.4 cases per 100,000 residents. For context, the overall U.S. incidence of Lyme disease in 2016 was 8.1 cases per 100,000 residents, down from 8.9 cases per 100,000 in 2015. The state with the highest incidence of Lyme disease, Maine, had 86.4 cases for every 100,000 residents. The 14 states on this list account for nearly 95% of the total reported Lyme disease cases in the U.S.

Many factors impact the prevalence of blacklegged ticks in a given year. The number of animals like mice and deer that the ticks feed on can affect the population. Weather also is a major driving force behind the blacklegged tick population, and by extension the incidence of Lyme disease. Ticks are inactive during the colder seasons, but if temperatures fail to drop in the winter, the ticks will stay active longer.

To determine the worst states for Lyme disease, 24/7 Wall St. reviewed Lyme disease data from the Centers for Disease Control and Prevention. The data included the incidence of Lyme disease in every state in 2016, the most recent year available, and the total number of confirmed cases by state. The share of the state population that lives in rural areas and the percentage of adults who are physically active are both for 2017 and come from County Health Rankings & Roadmaps, a Robert Wood Johnson Foundation and University of Wisconsin Population Health Institute joint program.