A speaker goes into the woods and tries to fit in. He sits on the ground because a boulder does, and lies back in the leaf litter to follow the sun the way an oak does. It's a gentle, almost humble way of belonging to the place.
Then the mood shifts. The woods are open and easy to see through, but the poem hints that something has been stripped away: everything except rocks and ferns has been "reduced to telltale pellets," which suggests deer have eaten the undergrowth. The speaker feels something on his face on the way home, and later a deer tick drops from his collar onto the supper table. He crushes it.
The final lines turn the small scare into a warning. A clear view looks pleasant, but here it signals sickness. The title fits: the tick is a symptom, and the speaker's calm, wary tone makes the ending quietly unsettling.