The first line of defense in the immune system is skin, mucus, saliva, tears, etc. The second line of defense causes a non-specific response, This response is rapid, short-lived and generic, and it lacks any memory of previous infections.
When the flu virus enters the body, it moves into the respiratory tract. There, the virus binds to the surface of cells and releases its genetic information (RNA) into the cell's nucleus, where the cell's genetic information is stored. As the virus replicates, or copies itself, it takes over the functions of the cell.
Common signs and symptoms of the flu include Chills and sweats, Headache, Dry persistent cough, Fatigue, Sore throat, etc.