Influenza is an acute, typically febrile, respiratory illness that occurs in outbreaks of varying severity, usually during winter months and mostly affecting the patients with extremes of ages (children and old people). Influenza viruses cause epidemic disease (influenza virus types A and B) and sporadic disease (type C) in humans [1]. During large-scale influenza A outbreaks, rates of hospitalization for lower respiratory tract disease increase for infants and children. This is also one cause of outbreaks of unexplained fever in infants less than 2 to 3 months of age [2].

Influenza Virus:

The influenza viruses are members of the Orthomyxoviridae family in the genus Influenzavirus. The type A influenza viruses are widely distributed in the animal kingdom (equine and swine viruses) and are particularly prevalent among birds. The influenza virion consists of an outer lipoprotein envelope surrounding central nucleocapsid material. The diameter of the virion is about 100 to 120 nm. The envelope is uniformly studded with spike-like projections that protrude from the viral membrane on its outer surface. The spikes are of two varieties: viral hemagglutinin (HA) and neuraminidase (NA). The inner surface of the envelope consists of the nonglycosylated viral membrane protein (M). The core ribonucleoprotein (RNP) encompasses a single-stranded RNA genome that is segmented. Each of the eight RNA fragments codes for one of eight viral polypeptides. The segmented nature of the influenza genome allows for exchange of RNA segments when two different influenza virions infect the same cell (genetic recombination). This property has great significance for the epidemiology of and control of pandemic influenza.

Classification of the influenza viruses as types A, B, and C is based on the antigenic properties of the internal RNP and M proteins. In addition to type-specific antigens, influenza viruses also possess strain-specific antigens that reside in the HA and NA moieties. A standard nomenclature has been devised to classify strains of influenza A viruses according to the antigenic characteristics of their HA and NA molecules (eg, H1N1 or H3N2). Humoral immunity is conferred by strain-specific antibodies directed at the HA and NA antigens, whereas type-specific antibodies do not neutralize viral infectivity. The degree of antigenic variation among influenza B viruses is less than among type A viruses. Influenza C is biochemically distinct from both type A and B.[3]