Earth is very dirty. There's no one place clean in earth. What seems clean is infested with microscopic living matter eating others. Even in the air we breath.
Everywhere in the world living beings are eating each other for survival.
The smartest and the dominamt ( humans) are eating the small and the bigger animals, but they are surrounded by the invisible microscopic even in the places they feel safe. like their bed. The comfort they built, is in the end a perfect nest for the thriving of the microscopic.
The smallest win them all. There's no threaten for what you can't see.