Microorganism

A microorganism (from the Greek:μικρός, mikros, "small" and ὀργανισμός, organismós, "organism") is a microscopic living organism, which may be single celled or multicellular. The study of microorganisms is called microbiology, a subject that began with the discovery of microorganisms in 1674 by Antonie van Leeuwenhoek, using a microscope of his own design.