While not every microorganism is dangerous, some microorganisms can cause disease and infections in humans and other living things. Microorganisms, also called microbes, also contribute to decomposition and spoilage of food, and they can leave toxic waste products behind even if they are eradicated from the contaminated material.
In some cases, microorganisms can have beneficial effects in small populations, but an overgrowth can cause issues. For instance, the bacteria that lives inside the human digestive tract, when balanced by other microbes in the body, serves an important purpose in the digestive process. However, an imbalance of nutrients or a weakened immune system can allow bacteria populations to grow, leading to an intestinal infection and potentially to a dangerous illness.