An acute viral infection involving the respiratory tract, occurring in isolated cases, in epidemics or in pandemics striking many continents simultaneously or in sequence. It is marked by inflammation of the nasal mucosa, the pharynx and conjunctiva and by headache and severe, often generalised myalgia. Fever, chills and prostration are common. Involvement of the myocardium and of the central nervous system occur infrequently. A necrotising bronchitis and interstitial pneumonia are prominent features of severe influenza and account for the susceptibility of patients to secondary bacterial pneumonia due to streptococcus pneumoniae, haemophilus influenzae and staphylococcus aureus. The incubation period is one to three days and the disease ordinarily lasts for three to ten days. Influenza is caused by a number of serologically distinct strains of virus, designated a (with many subgroups), B and c.