Also called the grippe or the flu, influenza is an acute, highly contagious infection of the respiratory tract that results from three types of Myxovirus influenzae. It occurs sporadically or in epidemics (usually during the colder months). Epidemics tend to peak within 2 to 3 weeks after initial cases and subside within 1 month.
Although influenza affects all age-groups, its incidence is highest in schoolchildren. Its severity is greatest in the very young, the elderly, and those with chronic disease. In these groups, influenza may even lead to death.
Transmission of influenza occurs through inhalation of a respiratory droplet from an infected person or by indirect contact such as using a contaminated drinking glass. The virus then invades the epithelium of the respiratory tract, causing inflammation and desquamation.
One remarkable feature of the influenza virus is its capacity for antigenic variation. Such variation leads to infection by strains of the virus to which little or no immunologic resistance is present in the population at risk. Antigenic variation is characterized as antigenic drift (minor changes that occur yearly or every few years) and antigenic shift (major changes that lead to pandemics). Influenza viruses are classified into three groups:
Type A, the most prevalent, strikes every year, with new serotypes causing epidemics every 3 years.
Type B also strikes annually, but only causes epidemics every 4 to 6 years.
Type C is endemic and causes only sporadic cases.