Respiratory diseases account for 95% of all illnesses that affect humans and are among the most studied diseases today. Their prevalence and incidence on all continents occurs in all age groups, and the multiplicity of their etiological agents means that ARIs (acute respiratory infections) are undoubtedly one of the world's biggest public health problems (Graham, 1991). Influenza A and B viruses are responsible for epidemics of respiratory diseases that occur worldwide, often associated with increased hospitalization and death rates (Mello et al., 1997a). The influenza C virus usually causes a much milder respiratory illness or no associated systemic symptoms and therefore does not have a significant impact on public health (Mims et al., 1995). The influenza A virus is distributed worldwide, affecting around 600 million people every year and is responsible for the frequent occurrence of outbreaks, epidemics and pandemics (Toniolo, 2001). This book presents an epidemiological study carried out by the respiratory virus group at the Federal University of Alagoas.