A Narrative Review of Influenza: A Seasonal and Pandemic Disease

Influenza is an acute respiratory disease caused by the influenza A or B virus. It often occurs in outbreaks and epidemics worldwide, mainly during the winter season. Significant numbers of influenza virus particles are present in the respiratory secretions of infected persons, so infection can be transmitted by sneezing and coughing via large particle droplets. The mean duration of influenza virus shedding in immunocompetent adult patients is around 5 days but may continue for up to 10 days or more—particularly in children, elderly adults, patients with chronic illnesses, and immunocompromised hosts. Influenza typically begins with the abrupt onset of high-grade fever, myalgia, headache, and malaise. These manifestations are accompanied by symptoms of respiratory tract illnesses such as nonproductive cough, sore throat, and nasal discharge. After a typical course, influenza can affect other organs such as the lungs, brain, and heart more than it can affect the respiratory tract and cause hospitalization. The best way to prevent influenza is to administer annual vaccinations. Among severely ill patients, an early commencement of antiviral treatment (