Within the respiratory system, air is first inhaled through the nose or mouth into the pharynx. From the pharynx, air is drawn through the larynx and trachea to make its way to the lungs.

Influenza is a viral infection of the respiratory tract that causes coughing, breathing difficulty, fever, headache, muscle aches and weakness. The virus is spread from person to person by inhaling infected droplets from the air.

The flu vaccine can also be administered as a nasal spray instead of the usual injection method. It is an alternative for healthy, non-pregnant people age 2 to 49 who want to be protected from the flu virus. Unlike the regular vaccine, it is a live virus. Therefore, it is best if the person receiving it does not have close contact with people who have a weakened immune system.