Microorganisms are found in every environment—in the air we breathe, the food we eat, the soil where food is grown, and the water we drink. Many microbes benefit humans, while only a few cause human diseases. Humans, being one of many organisms in the environment, affect and are affected by both living and nonliving components of the environment including its microorganisms. To control diseases, health scientists need to know how to control microorganisms in air, food, soil, and water. To do that they need to understand the roles of microorganisms in the environment. Like all other living things, microorganisms use water in metabolism, but they also live in water or very moist environments. Many form spores or cysts that help them survive periods of drought, but vegetative cells must have water. All microorganisms require some carbon source to maintain life. Most carbon entering living things comes from CO2 dissolved in bodies of water or in the atmosphere .