Watch Video | Listen to the AudioJUDY WOODRUFF: The arrival of spring is a most welcome sight to many of us, but it also brings a serious health problem, ticks. They can lead to Lyme disease in humans, which can be a serious chronic condition. Many suburban areas are trying to figure out just how to deal with an increasing number of ticks, which some blame on the growing population of deer in the U.S. Researchers are enlisted in this battle as well. Miles O’Brien has the story for our weekly segment on the Leading Edge of science. And a warning: Some of the images in this story might disturb some viewers. MILES O’BRIEN: This English manor in the woods near Boston is Denny Swenson and Wynn Swenson’s dream house. When they moved here 10 years ago, they were thrilled to see a lot of deer. DENISE SWENSON, Local Activist: And it was so exciting, because they’re such beautiful animals. They seemed harmless. Wow, we felt like we had arrived. MILES O’BRIEN: But, today, she is a more than a little wary of the deer. In fact, she believes controlling their numbers is the only practical way to stem the spread of Lyme disease. Humans are infected with Lyme from tick bites, and ticks like to feed on deer. DENISE SWENSON: There’s a trail there that the deer love. MILES O’BRIEN: This is deer country here? DENISE SWENSON: Yes, yes. We have tons of deer. MILES O’BRIEN: The state says that there are about 85 deer per square mile in the adjacent 7,000-acre Blue Hills Reservation; 10 per square mile is considered a density that could reduce the spread of Lyme. Deer were hunted nearly to extinction a century ago in the U.S. Today, thanks to legal protection and regulated harvests, the deer herd nationwide has exploded to more than 30 million. This creates a host of problems, including more than a million deer-car collisions each year in the United States. Suburbia offers the perfect habitat for deer to flourish, no human hunters or animal predators and a bounty of food. DENISE SWENSON: They’re coming all the way up to the front door to eat. They’re really hungry, so they kind of need the rhododendrons as much as I do. MILES O’BRIEN: Yes. This is kind of like the drive-through of a deer, right? DENISE SWENSON: Yes. This is the drive-through for them, yes. MILES O’BRIEN: Lyme is an infectious disease caused by a bacterium transmitted to humans by the bite of infected blacklegged ticks. Symptoms often begin with a distinctive bullseye rash and evolve into various neurological problems, including facial palsy, severe headaches, muscular aches, and sensitivity to light. Without antibiotic treatment, Lyme disease can lead to permanent motor, sensory and cognitive impairment. It is the most common tick-borne disease in the Northern Hemisphere. There are an estimated 300,000 cases a year all across the U.S. Both Denny and Wynn have had multiple cases of Lyme disease. DENISE SWENSON: Oh, I will never forget those headaches. I will never forget the pain sort of sweeping around my body. I felt like I was being invaded by some crazy thing. MILES O’BRIEN: The more she asked around, the more she got scared. DENISE SWENSON: We did a neighborhood survey, and 30 households responded, and we found 30 cases of Lyme disease. MILES O’BRIEN: You basically have a public health crisis here. DENISE SWENSON: Yes, these are epidemic level numbers. MILES O’BRIEN: A former journalist, Denny Swenson started researching. The Lyme disease bacterium lives inside white-footed mice, the so-called reservoir. Ticks that bite t...