Mycoplasma are a group of small, parasitic bacteria that lack cell walls and can cause diseases. They were first discovered by Louis Pasteur in animal cells as the pathogen that causes pleuropneumonia. Mycoplasma are the smallest known self-replicating organisms, were first cultivated in 1898 by French scientists Nocard and Knox, have no cell wall, can live without oxygen, and are sometimes called the "jokers of the plant kingdom" due to their unusual structure compared to other bacteria.