A microorganism is a microscopic organism, which may be a single cell or multicellular organism. The study of microorganisms is called microbiology, a subject that began with Antonie van Leeuwenhoek's discovery of microorganisms in 1675, using a microscope of his own design.
Microorganisms are very diverse and include all the bacteria and archaea and almost all the protozoa. They also include some members of the fungi, algae, and animals such as rotifers. Many macro animals and plants have juvenile stages which are also microorganisms. Some microbiologists also classify viruses as microorganisms, but others consider these as nonliving. Most[contradiction] microorganisms are microscopic, but there are some bacteria such as Thiomargarita namibiensis and some protozoa such as Stentor, which are macroscopic and visible to the naked eye.