A few hours after summiting Mount Katahdin in August 2014, Bennett Travers suspected something was wrong. After spending five months thru-hiking the Appalachian Trail, she was used to being tired. But this felt different. She was fatigued and nauseous. Instead of abating, the symptoms followed her home to Atlanta. "I couldn’t go up a flight of stairs and I could barely stand," she says.

Travers's doctor didn't initially suspect Lyme disease, but he gave her a blood test anyway. The results came back negative. "I was in the early window where Lyme didn’t appear, but we knew something wrong," Travers says. Since Lyme tests gauge antibody levels, rather than detect the bacterium that causes it, Borrelia burgdorferi, it can take a few weeks after an infection occurs for the immune system to produce enough antibodies to trigger a positive reading. Considering Travers's symptoms and the fact that she had just left the Northeast, where Lyme disease is endemic, her doctor prescribed three weeks of an antibiotic just to be safe. After that, Travers slowly improved and regained strength over the following weeks. By the spring, she was able to run a marathon.

"You're used to seeing insects and bugs on the trail, but you don't think anything of it," she says. "Lyme isn't something that you interpret as being a danger, especially when you've been out there for so long."

Ticks carrying Lyme disease are rampant in the forests of the northeast, and the Appalachian Trail goes straight through the thick of them. This year, a host of variables is coming together that could increase the likelihood of contracting the disease while hiking the trail, says Richard Ostfeld, a disease ecologist and senior scientist at the Cary Institute of Ecosystem Studies in Millbrook, New York.

In 2005, Ostfeld and his team compiled 25 years' worth of data into one of the most comprehensive field studies on the connections between blacklegged ticks (the main vectors of the disease) and environmental conditions, and how that relationship affects the risk of humans contracting the disease. "We expect the risk of coming into contact with a tick harboring Lyme disease will be higher in 2017 than in the average year, probably along large parts of the Appalachian Trail," he says.

"Lyme isn't something that you interpret as being a danger, especially when you've been out there for so long."

The core of the problem starts with a seemingly innocuous event: a bumper crop of acorns. During the summer of 2015, a spell of warm, wet weather in the northeast accelerated oak trees' ability to produce acorns, which happen to be the primary food source of the white-footed mouse, a rodent that's ubiquitous across the forests of the northeast. The mice are one of the main hosts of blacklegged ticks, and a carrier of Lyme. Ostfeld says the influx of mice in the middle of summer last year made it easy for ticks to find a host, which will lead to an abundance of infected nymphs in 2017. Ticks in the nymph stage are of highest risk to transmit diseases to humans because they’re hard to spot—no bigger than a poppy seed.

In addition to the bump in mice natality, the blacklegged tick has expanded its range. The bugs are now found in nearly 50 percent of counties in the U.S., whereas in 1998, they were only present in 30 percent. They're especially prominent in wooded areas east of the Mississippi.

Exactly which sections of the AT will be most affected is hard to predict, but much of the mid-Atlantic and New England regions—the Hudson Valley in particular—could be unusually fruitful incubators for the disease beginning in May and June, when nymphs emerge from their winter slumber. Most thru-hikers will enter those areas at that time.

Lyme disease is spreading across the U.S. at an alarming rate. Around 30,000 cases of Lyme disease per year are reported by state health agencies, according to the Centers for Disease Control and Prevention. However, the agency estimates the number of people diagnosed with the illness each year in the U.S. is roughly 10 times higher than the reported figures—closer to 300,000. (The overwhelming majority of infections occur in the Northeast, making the disease at least six times more common than yearly HIV/AIDS infections.)

If the onset is subtle, it’s easy for a hiker to simply brush off initial symptoms as a cold or a result of their long days and thus delay treatment.

But even that estimate could be far too low, according to Holly Ahern, an associate professor of microbiology at the State University of New York (SUNY) Adirondack. Ahern, who has studied the Lyme bacterium since 2010, figures new cases of Lyme disease are more likely double, closer to 700,000 per year. She says that's because the blood tests for Lyme, upon which the CDC’s statistics are based, have a sensitivity rate of less than 50 percent. "This means that for every two people who have Lyme disease, only one person will be diagnosed as having it. This leaves the other 50 percent without a diagnosis, and therefore without effective treatment," Ahern says. "It is not just the fastest growing vector-borne disease, it is already the second most common infectious disease, right after chlamydia, and ahead of gonorrhea."

A telltale sign of infection is a rash—often a bull’s-eye—near the bite. That sign is present in 70 to 80 percent of infected people, according to the CDC. A 2014 study by the Wilderness Medical Society surveying former and current thru-hikers found the group was bad at identifying the rash. The researchers also concluded that the risk of exposure is high for thru-hikers on the Appalachian Trail. Furthermore, section hikers, who take on chunks of the trail over several years, were less likely to seek medical treatment after exhibiting Lyme symptoms.

Early symptoms of Lyme include fever, chills, headache, fatigue, muscle and joint aches, and swollen lymph nodes. If the onset is subtle, it's easy for a hiker to simply brush off initial symptoms as a cold or a result of their long days and thus delay treatment. Often, a two- to three-week treatment of an antibiotic like doxycycline will quickly clear up Lyme disease. But if ignored or undiagnosed for prolonged periods, the illness can produce a wide range of symptoms, including short-term memory issues, facial palsy, arthritis, heart palpitations, and inflammation of the brain. At this stage, the illness can be difficult—and expensive—to treat. A Johns Hopkins study found that Lyme disease costs the U.S. healthcare system up to $1.3 billion annually.

So how many people have contracted Lyme while hiking the AT? It's hard to say.

According to an informal questionnaire provided by the Appalachian Trail Conservancy (ATC) to thru-hikers, nearly 5 percent of respondents in 2016 reported that they contracted Lyme disease while hiking. Since the ATC started recording that information in 2011, the rates have fluctuated between 3 and 6 percent. Since Lyme disease can be mistaken as another illness, or go undiagnosed, an exact number of people infected while hiking the trail is difficult to quantify.

What we do know is that the number of people attempting a thru-hike has more than tripled since 2007, with nearly 4,200 last year and even more expected this spring. The number of cases of Lyme recorded by the ATC have mirrored the increase in hikers. "It is not necessarily a steady increase year to year, although the numbers have certainly increased over the last few decades as the number of people on the trail have grown," says Laurie Potteiger, information services manager with the ATC.

A tick survey published in the journal Park Science in 2015 concludes that people are more likely to get bitten while hiking than while staying at shelters, and that ticks are more abundant at lower elevations, particularly below 1,600 feet above sea level. The greatest exposure to them is from May to July in states with low elevations, which include Maryland, Pennsylvania, New Jersey, New York, Connecticut, and Massachusetts. The AT in Vermont, New Hampshire, and Maine is generally too high in elevation to support many blacklegged ticks, though there's still the possibility of getting bit and contracting the disease there.

Many hikers aren't aware of tick prevention from the get go, Potteiger says, and some have never even put on a pack or set up a tent when they step foot on the trail. She says the group's greenness coupled with the prevalence of Lyme can increase the risk of contracting it. "Due to beginning in Georgia, where Lyme is still uncommon, and the time of year northbound thru-hikers are starting out, their exposure to ticks is minimal during the first several weeks, and developing Lyme strategies is not part of the learning process," Potteiger says. "By the time they’ve entered Lyme territory, it can be too late."

How to Prevent Tick Bites on the Trail

Treat Your Clothes

The most important and often overlooked prevention tool is for all hikers to have their clothes and gear treated with permethrin, an insecticide that kills bugs on contact, Potteiger says. You can purchase permethrin in spray form and apply at home, buy already treated clothing, or have clothes factory-treated by Insect Shield. She recommends tucking in your shirt into your pants and your pant legs into your socks to create a bug barrier. Spraying exposed skin with DEET can also help to repel them.

Bring a Pad

Hikers should also avoid sitting on logs or on the ground. It’s best to sit on a pad that’s been treated with permethrin, Potteiger says. "Ticks crawl into dark, moist areas, so when you do tick checks, pay attention to your armpits and groin."

Check Often

Removing a tick with tweezers within 24 hours can decrease the chances of contracting Lyme disease, so spotting them early is important. Potteiger says anyone spending time on the trail should know Lyme's symptoms so they're able to seek medical attention.

Change Your Route

Instead of starting in Georgia in March or April, as most do, Potteiger says to choose a route that minimizes the time spent in the Mid-Atlantic and southern New England from mid-May through July.