The etiology of influenza is unknown.

The portal of entry of the inciting cause is likewise undetermined.

The respiratory lesions, whether primary or secondary, are responsible for the high mortality of the disease.

The lesions of the respiratory tract peculiar to this disease and most frequently encountered are:

An acute tracheobronchitis associated with diffuse involvement of the pulmonary parenchyma.

Hyalinization of the epithelium of the air passages, and necrosis of the alveolar walls with extensive interstitial emphysema and occasionally with pneumothorax.

Dilatation of the terminal bronchioles.

Aplastic serofibrinous, and hemorrhagic, pneumonic exudates.

Necrotizing and organizing bronchiolitis, and pneumonia: lobar, lobular, peribronchial, interstitial.

The sequelÆ—obliterating bronchiolitis and bronchiolectasis.

Proliferation of alveolar and bronchiolar epithelium.

The extrarespiratory lesions of influenza are neither constant nor characteristic. Irrespective of localization, they have the same fundamental pathology characterized by:

Vascular damage.

Hemorrhage.

Organization with or without infection.

A basis for the interpretation of the respiratory lesions of influenza is offered by the analogous changes in the respiratory tract initiated by the inhalation of poisonous gases.

The respiratory lesions are dependent primarily upon the damage produced by the true etiological agent and the systemic capacity to compensate, and only secondarily upon invasion by the bacterial flora of the mouth and inspired air.