Infestation
an excerpt

They are here and living among us, and they have been since the dawn of time. When all is well, they function as symbionts although neither species is usually aware of the relationship. When there is balance between the species, there is peace and prosperity. We benefit from their lives and they benefit from ours. Throughout history, there have been a few humans who were able to perceive, and sometimes even see this strange parallel world. They have worked hard to understand these creatures and to maintain the balance and peace. When that balance is threatened or destroyed, like us, the symbionts will fight bitterly and in every imaginable way to preserve themselves as individuals and as a species. They too have a few rare individuals who are able to understand our world. And the time has come when our few and their few must stand together and help the rest of us to understand or all of existence is doomed.