A topographical account is given of the (mostly viral) infections that strike the upper airways and the lung parenchyma sites caused by bacteria, Mycoplasma, Rickettsiae and fungi. The different features of the three main syndromes that ensue: obstructive and restrictive ventilatory insufficiency, and alveolo-capillary block, are examined. The need for care in the administration of antibiotics is stressed, since germ resistance must be restricted as far as possible. In the case of chronic forms of lung disease, careful clinical surveillance and active therapeutic management are mandatory to prevent the establishment of irreversible, incapacitating consequences.