Eric at the Classical Values blog makes a persuasive case about hunting, roadkill, and Lyme disease's spread.
It is not mentioned here, but I have also seen the case made that fire suppression in Eastern forests has also created a more tick-friendly environment. Up through the 1600s, many Eastern forests were burned by Indians to create a more open environment for hunting, travel, and agriculture, resulting in fewer, larger trees. (On the west side of the Appalachian Mountains they were also able to encourage buffalo to graze and thereby be hunted.)
Lyme disease is not a big Colorado problem--yet. We get along fine with our hantavirus, bubonic plague, and West Nile virus. (Hat tip: Querencia)
Update: Here is a little more on the deer-car collision problem.
When I first moved to Fremont County, Colorado, in 1986, the district wildlife manager said that one deer or elk was killed nightly somewhere in the county.
Intelligent fencing is about all that works in high-risk areas.