The world is full of living things so small that most of us will never see them. Yet these invisible organisms have significant effects on human life – some good, some bad. Some microorganisms can cause infections that pose serious health risks, and left unchecked can grow into a plague. Other kinds of microorganisms are beneficial, and can fight infection or "eat" toxic waste. Microbiologists are the scientists who study these microscopic life forms, and their interactions with people, plants, animals, and the environment.