Any living thing that is invisible to the naked eye and can be seen only under a microscope is called a microorganism. They include BACTERIA, protists, and some fungi, such as yeasts. VIRUSES are usually included, but they are not really living organisms.
This is a single-celled organism found in the sea, fresh water, soil, and in or on other living things. Animal-like protists, called protozoa, get their energy by eating food. They include ciliates—protists that move by beating hairlike fibers called cilia. Some protozoa cause diseases such as malaria. Plantlike protists, called algae, make their food by photosynthesis, and include ocean phytoplankton and green pond algae.
French scientist Louis Pasteur founded the science of microbiology. He proved that microorganisms cause infectious diseases, developed vaccines, and discovered pasteurization (the heat-killing of bacteria in food).
Only a very powerful microscope can show the minute chemical package known as a virus. Many viruses cause disease. Viruses are active only once they have infected a living animal, plant, or bacterial cell.
The most abundant organisms on Earth, bacteria are found on land, in water, and in the air. Bacteria consist of one tiny cell. They have a protective cell wall, but, unlike other cells, lack a nucleus.