Mollicutes

The Mollicutes are a class of bacteria with no bacterial cell wall. "Mycoplasma" is the best-known genus.
They are very small, only about 0.2–0.3 μm in size, and have a very small genome. Most move about by gliding across host cells, but "Spiroplasma" are helical and move by twisting. 
Mollicutes are parasites of various animals and plants, living on or in the host's cells. Many cause diseases in humans. "Mycoplasma" and "Ureaplasma" stick to cells in the lungs or sex tubes like the urethra. "Phytoplasma" and "Spiroplasma" are plant pathogens with insect vectors. Bugs such as leaf-hoppers (Homoptera) are the vectors. They deliver the parasites when they stick their mouth-parts into a plant to suck its sap.