The organisms that do succeed in growing produce hydrogen peroxide which causes local tissue damage and a mitogen which stimulates lymphoid tissue in the lungs into growth. Mycoplasma disease does not show symptoms until the infection has been well established for some time and considerable tissue changes have occurred. Therefore the idea that treating the rat as soon as the symptoms occur is somehow 'getting in early' is entirely false. It is important to differentiate whether or not a rat has an upper or lower respiratory tract infection. In rats the disease tends to be chronic and even without treatment often the only symptom is rattling or noisy breathing or sneezing.
Pneumonia is rare and most affected rats have upper respiratory tract disease only.
Lower respiratory disease includes conditions ranging from mild bronchitis to pneumonia and sometimes emphysema. A rat with noisy or snuffling breathing is unlikely to have lower respiratory tract disease if it is otherwise healthy, eating and in good condition. It is far more likely to have sinusitis. Although the infection does not usually kill rats by itself they are rarely successful in getting rid of it from their system. The reason for this is not known.