Influenza viruses are causative agents of highly contagious, acute, viral infections of the respiratory tract. Influenza viruses are immunologically diverse, single-stranded RNA viruses. There are three types of influenza viruses: A, B, and C. Type A viruses are the most prevalent and are associated with most serious epidemics. Type B viruses produce a disease that is generally milder than that caused by type A. Type C viruses have never been associated with a large epidemic of human disease. Both Type A and B viruses can circulate simultaneously, but usually one type is dominant during a given season. Every year in the United States, on average 5%-20% of the population contract influenza; more than 200,000 people are hospitalized from influenza complications; and, about 36,000 people die from influenza-related causes. Some people, such as adults 65 years of age and older, young children, and people with certain health conditions, are at high risk for serious influenza complications. RSV is a causative agent of highly contagious, acute, viral infection of the respiratory tract in pediatric and elderly populations. Respiratory syncytial virus is a single-stranded RNA virus. Nearly half of all children become infected with RSV in their first year of life. It is also the major viral cause of nosocomial illness in children already hospitalized for other reasons. In the United States, RSV is estimated to be responsible for 73,400 to 126,300 hospitalizations annually for bronchiolitis and pneumonia alone among children younger than 1 year. In an analysis of U.S. viral surveillance and mortality data, respiratory syncytial virus (RSV) was reported as the most common viral cause of death in children younger than 5 years when compared to influenza A H1N1, influenza A H3N2, and influenza B. Among children hospitalized with RSV infection, the mortality rate is estimated to be as low as 0.3% to 1.0% and in the range of 2.5% to 4.0% for children with underlying cardiac or pulmonary disease Respiratory syncytial virus (RSV) is a respiratory virus that also infects the respiratory system and can cause an influenza-like illness. Most otherwise healthy people recover from RSV infection in 1 to 2 weeks. However, infection can be severe in infants, young children, and older adults. RSV is the most common cause of bronchiolitis (inflammation of the small airways in the lung) and pneumonia in children under 1 year of age in the United States, and is more frequently being recognized as an important cause of respiratory illness in older adults.