Human beings exist in and depend on a world of living organisms. The oxygen we breathe is produced by the photosynthetic activity of countless billions of individual organisms. The food that fuels our bodies comes from the tissues of other living organisms. The fuels that power our cars, trucks, and planes are carbon molecules produced by living organisms—mostly millions of years ago. Inside and out, our bodies are covered in complex communities of living unicellular organisms. Some help us be healthy, but others can invade our bodies and cause mild to serious diseases, or even death. These interactions with other species are not limited to humans. Ecosystems depend on thousands of complex interactions among the millions of species that inhabit Earth. Understanding biological principles is essential for us to lead healthy productive lives and to be good stewards of our planet.