Type A is responsible for periodic worldwide epidemics; types A and B cause regional epidemics during the winter.
The recurring pattern of the influenza viruses is due to their ability to exhibit variation in surface antigens. Two phenomenon account for this variability:

Antigenic drift is due to mutations in the RNA that leads to changes in the antigenic character of the H and N molecules. Antigenic drift involves subtle changes that may cause epidemics but not pandemics.

Antigenic shift is due to rearrangement of different segments of the viral genome that produces major changes in the antigenic character of the H and N molecules. Antigenic shift usually occurs in animal hosts and is responsible for producing both epidemics and pandemics.

Orthomyxoviruses contain a single stranded, negative RNA genome divided into 8 segments.
The viruses have a lipid bilayer envelope with surface glycoproteins (hemagglutinin and neuraminidase)

Viral attachment is mediated by the hemagglutinin. The virus enters host cells by pinocytosis and uncoating occurs by fusion of the viral envelope with the membrane of the vacuole. The RNA is capped and replication proceeds in the nucleus. The progeny are released by budding and cell death ensues.

There are 3 viral antigens of importance: the nucleoprotein antigen that determines the virus type (A, B or C), the hemagglutinin (H) antigen, and the neuraminidase (N) antigen. The H and N antigens are variable.

Pathogenesis

Transmission of disease is airborne. The viruses deposit in lower respiratory tract, their primary site is the tracheobronchial mucosa.

Neuraminidase produces liquefaction, which leads to viral spread.

Respiratory symptoms include a cough, sore throat and nasal discharge.

The extent of respiratory tract cell destruction is a probable factor in the disease.

Severe complications include pneumonia (viral or bacterial)

Host Defences

IgA is produced in the upper respiratory tract and IgG is produced in the lower respiratory tract. These antibodies are directed primarily against the hemagglutinin and neuraminidase.

EPIDEMIOLOGY:
Influenza displays a typical pattern: school children bring the disease home and infect siblings and parents.
Epidemics usually last from 3-6 weeks and the highest attack rates are for 5-19 year olds (generally Type A).

DIAGNOSIS :Influenza usually displays a sudden onset with fever, malaise, headache, muscle aches, sore throat, cough and, generally in winter. The presence of disease in the community (i.e. epidemiology) is helpful in diagnosis.