Influenza Virus Causes

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Influenza is an acute respiratory illness caused by infection of influenza A and B viruses. The disease can affect both the upper and lower respiratory tract and is often followed by systemic signs and symptoms, such as: sudden onset of fever, chills, non-productive cough, myalgias (muscle pain), headache, nasal congestion, sore throat, and fatigue. (Cox et al.1998). Influenza viruses evolve continuously, challenging mammalian and avian hosts with new variants and causing complex epidemic patterns with regard to age, place, and time. Human influenza viruses cause disease through a variety of direct and indirect pathological effects. The direct effects include destruction of infected cells, damage to respiratory epithelium, and immunological responses that cause general malaise and pneumonia. Indirect effects of infection include secondary bacterial infections due to the tissue damage and other disease such as cardiovascular disease, renal disease, diabetes or chronic pulmonary disease (Schoenbaum S.1996). In the USA, clinical illness affects 5–20% of the population and asymptomatically infects a larger number (Noble G.1982). Infants, who are exposed to influenza epidemics as a novel antigenic challenge after maternal antibodies decline, may have attack rates as high as 30–50% in their first year of life, depending on the frequency of contacts with older siblings (Glezen et al.1997). For reasons, influenza viruses cause epidemics in the northern and southern hemisphere during their respective winters. In the tropics, the timing of activity is less defined, with sometimes year-round circulation or bi-seasonal peaks during the year (Viboud et al.2006).
There are three types of influenza;
Seasonal influenza: Spread of seasonal influe...

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...creased risk for complications of influenza.
Although the absolute number of deaths does not always differentiate a pandemic from severe non pandemic seasons, the age distribution of influenza-related deaths has noticeably illustrious pandemics from the interpandemic seasons instantly preceding them. Persons under 65 years of age accounted for half of the influenza-related deaths in the United States during the 1968–1969 influenza pandemic, but far smaller proportions during the decades following the pandemic.
A similar pattern was seen in the United States for all three pandemics of this century (Simonsen et al.1998). Thus, younger persons were at a 20-fold high risk for influenza-related mortality during a pandemic, whereas the elderly faced roughly the same risk during a pandemic as during later severe inter pandemic seasons subjected by the same virus subtype.

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