And not for a nice reason, either. The little White Footed mouse is apparently "not fastidious" and doesn't mind ticks attached to its body. It also has a crap immune system. This means that the Borrelia burgdorferi it often carries gets into the ticks, which, when they are grown up, bite humans and give us Lyme Disease. They eat acorns, so a good acorn year is followed by a good mouse year, which is followed by a good (for B. burgdorferi) Lyme year.
Washington Post has the full story (text here).