Like much of the South, in which extreme Southern Illinois resembles in some ways, there is the risk of running into a few different species of poisonous snakes: cottonmouths water moccasins, copperheads, and timber rattlers. Be vigilant and use common sense when hiking; some parks may well have more of a population or frequency of encounters then others. In fact, a portion of the National Forest is closed off each year to allow for the ritual snake migrations that take place there protecting you and the snakes from harrassment.
fees/permits
There are no fees to enter the Shawnee National Forest.