A genus of micro-organisms which differ from typical bacteria in that they lack a rigid cell wall. They are responsible for widespread epidemics in cattle and poultry. For a long time the only member of the genus known to cause disease in humans was Mycoplasma pneumoniae which is responsible for the form of PNEUMONIA known as primary atypical pneumonia – particularly common in children, for whom it is the single most common cause of the disease when contracted out of hospital. Another, Mycoplasma genitalium, has now been isolated and is responsible for certain cases of non-gonococcal urethritis. Mycoplasma infections respond to TETRACYCLINES.