Lyme disease is the most common tick-borne illness in North America and Europe. Lyme disease is caused by the bacterium Borrelia burgdorferi. Deer ticks, which feed on the blood of animals and humans, can harbor the bacteria and spread it when feeding.

You’re more likely to get Lyme disease if you live or spend time in grassy and heavily wooded areas where ticks carrying the disease thrive. It’s important to take common-sense precautions in areas where Lyme disease is prevalent.

Lyme disease is a bacterial infection that is transmitted mainly through Ixodes ticks, more commonly known as deer ticks, on the East Coast and black-legged ticks on the West Coast of the U.S.A. According to Lymedisease.org, the symptoms of Lyme disease—which include but are not limited to flu-like symptoms, a bull’s-eye shaped rash, and/or Bell’s palsy (drooping of the face)—are often misdiagnosed as other conditions, earning Lyme disease the title of “The Great Imitator.” According the Centers for Disease Control and Prevention (CDC), an estimated 300,000 new cases of Lyme disease are diagnosed across the country each year, more than the annually diagnosed number of breast cancer or HIV/AIDS cases.

The Increasing Threat

In recent years, Lyme disease has appeared to be a growing threat to public health. According to the CDC, there were just over 10,000 confirmed cases of Lyme disease in 1995. In 2013, that number grew to over 25,000, with an additional 10,000 probable (yet unconfirmed) cases. A leading theory behind the increasing prevalence of Lyme disease revolves around forest fragmentation.

Forest fragmentation occurs when larger swaths of forest are broken up into small woodlots (less than one hectare each) in order to make room for agricultural fields or suburban developments. When larger forests are broken up, their ecosystems undergo significant change. Small woodlots cannot sustain large or mid-sized predators and therefore become ideal habitats for small mammals. Mice, especially, thrive in small woodlot areas. Small woodlots cannot sustain the natural predators of mice (long-tailed weasels, red and gray foxes, coyotes). Additionally, mice have proven to be able to adapt to life in small woodlot areas in ways that some other small mammals (chipmunks, squirrels) have not. Therefore, in many small woodlots (which accounts for much of the wooded areas in suburban and rural communities) mice are the most populous mammals.

Though deer and blacklegged tick bites are the main sites of Lyme disease transmission in humans, mice are the main carrier of Borrelia burgdorferi, the bacterium responsible for Lyme disease. Large populations of mice in small woodlot areas attract and sustain large tick populations, which feed on the infected mice. According to the National Science Foundation, small woodlot areas have approximately three times the number of ticks of larger forest fragments, and seven times the number of infected ticks. Though the mice are unaffected by Borrelia burgdorferi, ticks acquire and the bacteria while feeding on the mice and later transmit the bacteria to their human host.