symptoms are like those of a common cold. In more severe cases, symptoms typically start suddenly with chills and fever (up to 39 to 39.5ºC) prostration and generalized aches and pain (most pronounced in the back and legs). Headache may be is prominent, often with photophobia and retrobulbar aching. Respiratory tract symptoms may be mild at first, with scratchy sore throat, substernal burning, non productive cough, and some times coryza. Later, the lower respiratory illness becomes dominant; cough can be persistent and productive. In severe cases, sputum may be bloody. Nausea and vomiting may occur in children. After 2 to 3 days, acute symptoms subside and fever usually resolves
Abnormal lung clearance and altered bronchiolar air flow can be demonstrated, and, in asthmatics, attacks are frequently precipitated by weakness and fatigue. Fulminant pneumonia is rare, but when it occurs, death may ensue in as little as 48 hours.
Secondary bacterial infection of the bronchi and lungs, most commonly pneumococcal or staphylococcal, is suggested by persistence or recurrence of fever, cough and other respiratory symptoms in the 2nd week. When pneumonia develops, cough and fever worsen, purulent or bloody sputum may be produced, and pleuritic chest pain may occur.
Encephalitis, myocarditis, and myoglobinuria are infrequent complications of influenza and, if present, usually occur during convalescence. Virus is rarely recovered from organs outside the respiratory tract, and a specific role in the pathogenesis of the extra- pulmonary diseases cannot be positively established. However, an increased incidence of such disease regularly follows influenza A pandemics. Reye's syndrome, characterized by encephalopathy, fatty liver, hypoglycemia and lipidemia, has been prominently associated with epidemics of influenza B, particularly in children who have ingested aspirin.