Influenza (flu) is medically defined as a contagious viral infection mainly affecting the nose, throat, airways, and lungs. There are several strains of viruses blamed for causing flu. Each strain can mutate and change frequently. Type A is most common and found in many animals as well as humans. It’s credited with causing the worst kinds of flu. Type B is said to cause milder cases of flu. Type C hasn’t been associated with causing the annual plague of flu. A common cold is not considered to be flu. Stomach flu s not influenza.