The respiratory tract is one of the main portals of entry into the human body for microbial pathogens. On average, a human takes about 20,000 breaths each day. This roughly corresponds to 10,000 liters, of air. Suspended within this volume of air are millions of microbes of terrestrial, animal, and human origin including many potential pathogens.A few of these pathogens will cause relatively mild infections like sore throats and cold. Others , however, are less begin.According to the World Health Organization, respiratory tract infections such as asthma, influenza, and pneumonia were responsible for more than 4 million deaths worldwide in 2012