Influenza is an acute respiratory illness caused by infection with influenza A and B viruses. The illness 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, nonproductive cough, myalgia (muscle pain), headache, nasal congestion, sore throat, and fatigue. (Cox et al. 1998). Influenza viruses continually evolve, challenging mammalian and avian hosts with new variants and causing epidemic patterns that are complex in terms of age, location, and time. Human influenza viruses cause disease through various direct and indirect pathological effects. Direct effects include destruction of infected cells, damage to the respiratory epithelium, and immunological responses causing general malaise and pneumonia. Indirect effects of infection include secondary bacterial infections due to tissue damage and other diseases such as cardiovascular disease, kidney disease, diabetes or chronic lung disease (Schoenbaum S.1996). In the United States, clinical disease affects 5 to 20% of the population and asymptomatically infects many more (Noble G.1982). Infants, who are exposed to influenza outbreaks as a new antigenic challenge after maternal antibodies wane, can have attack rates of up to 30–50% during their first year of life, depending on the frequency of contact with their older siblings (Glezen et al. 1997). For this reason, influenza viruses cause epidemics in the northern and southern hemispheres during their respective winters. In the tropics, the calendar of activity is less defined, sometimes with year-round circulation or bi-seasonal peaks during the year (Viboud et al. 2006). There are three types of flu: Seasonal flu: spread of seasonal flu. .... middle of article ......increased risk of complications from the flu. Although the absolute number of deaths does not always differentiate a pandemic from severe non-pandemic seasons, the age distribution of influenza-related deaths presents remarkably illustrious pandemics. interpandemic seasons that instantly precede them. People under 65 accounted for half of flu-related deaths in the United States during the 1968-1969 flu pandemic, but much smaller proportions in the decades following the pandemic. A similar trend has been observed in the United States for all three pandemics this century (Simonsen et al. 1998). Thus, younger people faced a 20 times higher risk of influenza-related mortality during a pandemic, while older people faced about the same risk during a pandemic as in subsequent severe inter-pandemic seasons subject to the same virus subtype..