INFLUENZA

Acute respiratory tract infection accompanying systemic symptoms are fever, malaise, coryza and myalgia. It is caused by influenza virus belonging to the orthomyxoviridae group. Three types of Influenza viruses occur in humans i.e. Influenza A, B and C All the known pandemics were caused by influenza A strains as it is known to change its genetic makeup by antigenic “shift” and “drift”. Influenza B is comparatively a genetically stable virus without any animal reservoir and Influenza C causes a milder disease. There is one more type, influenza D which occurs exclusively in cattle. The type A viruses are further divided into various subtypes based on the hemagglutinin “H” and neuraminidase “N” antigen expressed on their surface. There are 18 subtypes of hemagglutinin and 11 subtypes of neuraminidase. Influenza epidemics affect 10-20% of the global population on an average each year and are typically the result of minor antigenic variations of the virus or antigenic drift, which occur often in influenza A virus. On the other hand, pandemics which are associated with higher mortality appear at longer and varying intervals (often many decades) as a consequence of major genetic reassortment of the virus (antigenic shift) or adaptation of an avian or swine virus to humans (as with the pandemic H1N1 virus of 1918).