● At first haemophilus influenza was considered as the causative agent for influenza but after the research it has been found that it caused various other types of infections but not influenza. Influenza was caused by some flu virus which was first isolated from pigs in 1931 and from humans in 1933.(4)
● The 1918 influenza pandemic was the most severe pandemic in recent history it was caused by an H1N1 virus with the genes of avian origin although there is not universal consensus regarding where the virus originated it spread worldwide during 1918 and 1919. (3)
● It was first identified in military personnel in spring 1980 it is estimated that about 500 million people or one third of the world's population became infected with this virus.(3)
● The number of deaths was estimated to be at least 50 million worldwide with about 6,75,000 deaths occurring in the United States. (3)
● Mortality was high in people younger than five years old ,20 to 40 years old and in 65 years and older. The high mortality in healthy people including those in the 20 to 40 year age group was a unique feature of this pandemic. (3)
● There was no vaccine to protect against influenza infection and no antibiotics to treat secondary bacterial infections that can be associated with influenza infection control efforts worldwide were limited to non-pharmaceutical intervention such as isolation quarantine good personal hygiene use of disinfectants and limitations of public gathering which were applied unevenly.(3)
● In the northern and southern parts of the world outbreaks occur mainly in the winter while around the equator outbreaks may occur at any time of the year .In the northern and southern parts of the world outbreaks occur mainly in the winter while around the equator outbreaks may occur at any time of the year.(7)
● In the 20th century three influenza pandemics occurred Spanish influenza in 1918 where the death toll ranged from 17- 200 million deaths, Asian influenza in 1957- 2 million deaths and Hong Kong influenza in 1968 -1 million deaths.
● The world health organisation declared an outbreak of a new type of influenza A or H1N1 to be a pandemic in June 2009.
● influenza may also affect other animals including pig horses and birds.(9)
● The name “influenza” originated in 15th century Italy, from an epidemic attributed to
“influence of the stars.” The first pandemic that fits the description of influenza was in
1580. At least four pandemics of influenza occurred in the 19th century, and three occurred in the 20th century. The pandemic of “Spanish” influenza in 1918–1919 caused an estimated 21 million deaths worldwide. The first pandemic of the 21st century occurred in 2009–2010. Historically, influenza viruses of three HA subtypes (H1, H2 and H3) have acquired the ability to be transmitted efficiently between humans. Currently, influenza viruses of the H1 and H3 subtype co-circulate in humans, however influenza viruses of the H2, H5, H6, H7 and H9 subtype are also considered to represent a pandemic threat. In 1997, a large outbreak of highly pathogenic avian influenza (HPAI) H5N1 virus in poultry in Hong Kong resulted in the first documented cases of direct transmission of HPAI H5N1 virus from poultry to humans, with a fatal outcome in 6 out of 18 cases [17]. As a result, this outbreak warranted the mass culling of 1.5 million chickens. In 2003, a large outbreak of an HPAI H7N7 virus in poultry in the Netherlands resulted in 89 cases of human infections, one of which was fatal [21]. HPAI H7N7 virus displayed an unusual tissue tropism; the virus targeted the conjunctiva, resulting in conjunctivitis, a symptom rarely reported for other influenza virus subtypes.(1)
Prior exposure to immunogenic peptides found in human influenza A viruses may influence the age distribution of cases with avian influenza H5N1 and H7N9 virus infections
