The fear of ticks, and of the Lyme disease these bloodsuckers carry, is well founded: roughly 30,000 cases of Lyme are reported to the U.S. Centers for Disease Control and Prevention every year. Because most cases go unreported, the true toll is more like 300,000, the CDC estimated in August. The new figure “confirms that Lyme disease is a tremendous public health problem,” Paul Mead, the CDC's chief of Lyme epidemiology and surveillance, said at the time.

As investigators struggle to explain Lyme's prevalence, some have shifted focus from the long-maligned deer that carry adult ticks to a smaller culprit. The white-footed mouse, which hosts immature ticks, is especially efficient at passing the Lyme-causing bacterium Borrelia burgdorferi from one generation of ticks to the next.

The mouse is also an opportunist that thrives where other species cannot. As human development fragments forests into smaller patches, white-footed mice increase in density even as other animals disappear. “It is an animal weed,” says Felicia Keesing, a professor of biology at Bard College. “Anything that causes a surge in the population of these mice is something to watch.” Predator removals can cause just such a surge. A 2012 study found that Lyme incidence in recent decades coincided not with deer abundance but with declines in the population of red foxes, which eat mice and other small mammals.

Testing ideas about Lyme in the wild is exceedingly difficult. As a result, some researchers contend that the best protection is a diverse animal population that controls or dilutes the effects of white-footed mice. Others argue that targeting deer, which allow ticks to reproduce, remains the better strategy. In the meantime, as researchers debate the relative importance of the species implicated in Lyme disease, B. burgdorferi is doing just fine.