Brainworms wiggle into a snail, which is eaten by a deer. They travel through the stomach, into the nerves, up the spinal cord, and into the brain where they lay their eggs. The eggs hatch and migrate to the lungs. Then the deer coughs them up and reswallows them. The brainworms then find themselves back into the stomach where the cycle starts again.
Filarial worms cause such things as elephantitis and heartworms in dogs.