The major conflict in Mrs. Frisby and the Rats of NIMH is Man (non-human protagonist) vs. Nature. When Mrs. Frisby's son Timothy comes down with pneumonia, she faces a serious choice: try to escape to the woods to avoid plowing, or stay in hopes that their home will be missed by the plow. In the world of the animals, human activity is as much a natural event as rain or snow; they cannot affect Man's activities, and so they try simply to survive them.

When winter is over, they must move out of the garden and back to the meadow or pasture. For as soon as the weather allows, Farmer Fitzgibbon's tractor comes rumbling through, pulling the sharp-bladed plow through the soil...
(O'Brien, Mrs. Frisby and the Rats of NIMH, Amazon.com)

Added to the danger of the plowing is the cold weather, which will further harm Timothy's health. Mrs. Frisby fights for her children against other animals, against natural hazards, and against the uncaring actions of humans, who don't even know she is there. In her struggle against the world, she shows her individual bravery and willingness to sacrifice for her family.