A microorganism or microbe is a microscopic 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 the 1670s by Antonie van Leeuwenhoek. Robert Hooke coined the term "cell" after viewing plant cells under his microscope. Later, in the 19th century, Louis Pasteur found that microorganisms caused food spoilage, debunking the theory of spontaneous generation. In 1876 Robert Koch discovered that microorganisms cause diseases.